Hi gdungan,
Welcome to the Mattress Forum! 
I appreciate you taking the time to monitor things and add a comment.
Yes, 4 lb polymer density (not a density with added fillers) is what I’m comfortable recommending as a minimum for a durable layer of memory foam (especially in an upper layer that receives the most mechanical stress) under normal use, unless someone is in a more restricted budget range (where they might choose to go with a lower density foam), or conversely unless someone is in a higher weight range (above 200 pounds or so), where I would recommend 5 lb memory foam. While I mention my reasoning for these guidelines in various areas of the site, I appreciate you bringing up this topic and allowing me to address this again.
First off, the information on the site and in the forum comes from many sources. Some of it comes from thousands (literally) of hours of conversations with dozens of mattress manufacturers all around the country who in many cases have made mattresses for decades. They are well aware of the different materials that are available to them and the specs, advantages, and disadvantages of each material, and they have “real world” experience how these different materials hold up over time.
Part of it comes also from having come to know what you could call “insiders” in the industry who have many years of experience in different parts of the industry, including foam pourers, are well aware of the trends that are happening, and why different manufacturers take the approach they do in manufacturing and pricing their mattresses.
Part of it comes from thousands of hours of research (again literally) into the properties and specs of the different materials that are used in mattresses and what they really do as opposed to what various interests would have people believe they do. There are many technical sources on the internet but they usually involve going much deeper into various searches and then taking the time to read (and understand) some very difficult and technical reading. Sources like patent applications, SEC 10K forms, industry publications and sites, foam manufacturers, and many others are also valuable sources of more accurate information than most people are willing or have the time to pursue.
Part of it also comes from an understanding of how different types of manufacturing techniques and layering patterns interact with each other and with the people sleeping on them. This information too has come from many sources both online and on many hours of conversations.
Part of it come from many hours of personal testing of mattresses with different types of layering, construction, and materials both by myself and others to “translate” various ideas and knowledge into practical terms.
Regarding density, when it comes to polyfoam and memory foam, the polymer density of the foam (relative to other foams of the same type) is the most significant factor in its durability. This is a matter of foam chemistry. Density is primarily a matter of how much material is in the foam and the composition of that material vs air content. The primary benefit of density is durability. Higher density memory foams in the 5 lb range and higher will be the most durable, medium density memory foams in the 4 lb range will be less durable but may be preferable for some people because of the differences in how some of them feel and perform, and I would avoid any memory foam that is less than 4 lb density unless you are in the lowest budget ranges because of potential durability issues. Ultra high density memory foams in the 6 lb range and higher may be slightly more durable yet but the benefits of greater durability begins to level off above this range (see page 4 here or here for a bit more information about polyfoam density).
Foam additives can be used either as a filler to lower the cost of the foam, increase fire retardancy, make production easier, or they can be added to a foam to change or improve its properties (such as gel materials or some types of filler to improve stiffness) so when there are fillers added to memory foam or polyfoam it’s difficult to know the effect it will have on durability. These fillers can include silica, carbon black, calcium carbonate, various clays and even gel particles, all of which can give the memory foam a higher total density than the polymer density of the foam before the fillers are added.
While additives can increase density and in some cases improve support (especially at the very end of compression), they can also reduce foam durability and make the foam more likely to tear. Being an additive and not part of the cell structure, they can act as abrasives that can break down cell walls. This is why it is important to look at the polymer density when considering foams. Of course chemists are always striving to improve foams and these technologies, but I’m still very comfortable with my recommendations based upon this and the feedback and information I receive on a continual basis from my many industry contacts. So in essence … polymer density remains the single biggest factor in memory foam durability. There are other factors (such as the use of different chemicals and polymers) but these are secondary.
In the case of memory foam there are also other factors that are independent of density that people consider.
Breathability: this is determined by how open celled the foam is. The more the cells are open … the easier the air moves through the material which can make the foam faster reacting. While it’s true that in general … lower density tends to have more open cells … this is only a loose relationship and high or low density foams can be either more or less open celled. More open celled foams that allow for more air circulation tend to sleep cooler.
Viscosity/elasticity ratio: Viscoelastic or memory foam is partly viscous (meaning it flows away from pressure like a liquid) and partly elastic (meaning it compresses under pressure, stores energy and bounces back). Any density of memory foam can have either more or less viscosity or more or less elasticity. In general though … the chemicals that give the foam more viscous qualities (memory foam like) are heavier so lower density memory foams tend to have less “memory” and tend to be faster reacting.
Temperature sensitivity: Viscoelastic foam goes through a transition from elastic to viscous within a certain temperature range. Foams that are more temperature sensitive will be firmer when they are cooler and softer when they are heated by the body or the environment. Humidity also plays a role here (higher humidity softens it). Memory foams that are less temperature sensitive will tend to have less of a range of softness and firmness. foams that are more temperature sensitive will be both firmer and softer depending on the degree of softening and transition into viscosity from elasticity. You can read a little more about the benefits of the various types of gel and gel foam materials (including gel memory foam) in post #2 here . The cooling benefits of gel tend to be temporary and can help when you are going to sleep but not so much over the course of the night. A bigger benefit of gel when it is added to memory foam is that it can improve the support qualities of the memory foam and decrease its tendency to get softer over the course of the night (called “foam creep”).
Softness: This is determined by the formula of the foam, its temperature sensitivity, and of course is also dependent on body and external temperature, humidity, and time spent on the foam. Some foams have a range of very firm to very soft while some start off softer and never get quite as firm. All memory foams, even the ones that are firmer than others (they range from an ILD of about 8 to about 18 although ILD is not that significant with memory foam), are considered to be soft foams and the biggest variable is how long it takes for them to get soft. This is why memory foam cannot be used as a support material in a mattress (it’s only used in the top layers) and there is always a different material used under the memory foam to support the weight of the body. Memory foam is a slow responding pressure relieving material only. Because memory foam is time dependent and also depending on the speed of compression … It takes more pressure to press down on memory foam that it exerts coming back up and ILD is measured after a time delay which is another reason it can be so misleading with memory foam.
Response rate: Like any liquid … firmness and softness will depend on how quickly you compress it. If you press quickly on water or honey for example … it takes “time” for it to flow away and will feel firmer than if you press it much more slowly. Denser foams tend to be “slower” in response although the other factors mentioned previously can modify this as well.
Pressure relief: This is connected to both softness and density because higher density foams can “melt” into shapes that more closely fit the body which means they distribute weight better and lower the pressure around pressure points. They can take more time to “melt” though so with initial compression or with movement they can feel much firmer. This is why some memory foam companies will say that their 6 lb foam is the most “conforming” even though their 4 lb foam is the “softest” (gives more with initial pressure).
These are just a few of the factors that can be included in a foam’s composition and which results in different memory foams that “feel” very different … even if the density (durability) is the same.
And while the individual components within a mattress are of course important (they are the “building bricks of the mattress”), there are also other factors involved that will dictate the overall durability of a foam layer in a finished mattress, and the completed mattress, in general.
While it discusses all types of foams, there is a bit more listed here regarding the durability of foams within a mattress in post #2 here. Post #3 here discusses some factors as to how long a mattress will last. How long a mattress remains suitable for the comfort and support needs and individual preferences of a specific person is discussed in post #4 here. The Mattress Durability Guide on the site has everything in one spot.
There is also a brief outline of recommendations I make for buying a memory foam mattress in post #10 here.
So as is evident, over the years there’s been quite a bit of information compiled here on the site regarding foam density, quality, performance and overall mattress durability, and in following, my basis for the recommendations that I make.
Of course, everyone is free to believe what they wish to believe and take my advice and use it as they will. I’m always happy to discuss and provide the basis for my reasoning.
Thanks again for posting!
Phoenix