Hi joebk,
this would really depend on the “feel” and performance you are looking for and what the topper is being added to. A topper that has a combination of polyfoam and memory foam will have a feel that is more in between a fast response (latex or polyfoam) and slow response (memory foam or gel memory foam) material. Regardless though … I would look for high quality materials rather than just the generic material itself. All materials can be high or low quality. For those that are looking for more of the slow response memory foam feel … then a 3" layer of memory foam will feel more like what they are looking for than say a topper which has 2" of memory foam and 2" of polyfoam. I personally like the feel of a combination of slow and fast response materials but it would be important to know how a combination like this felt to you because each person’s needs and preferences can be very different.
There are many factors involved in how long a mattress or a topper will last for any particular person. Some of the many variables are in post #2 here. Because of all the variables … there is really no way to know for sure how long it will last for any particular person except to say that 4 lb memory foam will soften more and is less durable than 5 lb memory foam. As a topper though … and also depending on all the many factors including your own weight, how thick it was, what is over and under it, and how much “room” you have for the foams in your sleeping system to soften and still provide you with good pressure relief and support … I would be surprised if a 4 lb topper lasted for 10 years and probably half or that or a little more would be a reasonable expectation. For some people who have little room for a mattress to get softer and still have good support, it may last a few months (even the initial break in period may be more softening than they can tolerate for their spinal alignment) while for others who can tolerate much more foam softening because of the other factors involved … it may last 10 years (although this would also not be the norm).
Again depending on all the other factors involved … a good “true” HR polyfoam will generally last longer than a 4 lb memory foam yes and will begin to approach latex in its durability.
The “cloud like” feeling is not unique to memory foam (it’s usually a result of good alignment and even pressure distribution which can happen with any material) but the memory foam version of a “cloud like feeling” would probably need 3-4" of memory foam. Of course this would also depend on the type of memory foam not just the density (there are hundreds of different formulations) and on what is over or under the memory foam as well. All the materials interact together and each layer will affect every other layer to greater or lesser degrees. If you have a mattress that already has several inches or more of softer materials in the upper layers … then adding enough memory foam to get the “feel” you want may also compromise alignment. Generally thinner soft toppers are less “risky” in terms of alignment and it’s always a tradeoff between the feel you want and the alignment/support you need.
Comfort is what you feel when you first get into bed at night (and this is what people usually pay the most attention to). Alignment is what you feel when you wake up in the morning (people generally pay less attention to this because spinal alignment is not as “obvious” or as easily tested as “comfort”) and durability/quality is what people will feel a year or more down the road when the foam softens (and to know the relative durability of a topper or a mattress compared to other materials or quality levels you need to know the “quality” specs because you can’t “feel” quality).
Most local foam shops would carry various versions of polyfoam and fabric shops that were doing upholstery would probably have some as well but you would need to ask if they carried HR quality because some will and some won’t. Some may not even know what you are talking about 
Again this depends entirely on the body type, sleeping style, the mattress it is being added to, and the many other variables involved. Each person can be different. Heat issues also have many different factors involved in them. One of these is the breathability of the foam, one is how much you sink into the mattress, and one is the layers that are over or under the mattress. As a general rule … polyfoam is more breathable than memory foam but each material has a range of properties depending on the formulation of the foam. Lower density foams are often more breathable than higher density foams as well (they contain more air and less materials). Convoluted foams have a different compression curve than non convoluted foams but they will also generally be less durable than a non convoluted layer of the same type (they will be more subject to repeated deeper compression and mechanical wear).
There is more about the factors involved in the sleeping temperature of a mattress in post #2 here and information about temperature regulation that is more specific to memory foam in post #6 here.
One of the things that can be most frustrating (or an interesting challenge for some) about building a mattress or choosing materials based on “specs” is that there are really no simple or black and white answers to most of the questions people ask because the usually involve multiple interacting factors or layers. This is where the experience and knowledge of a manufacturer or better retailer and their knowledge of a specific mattress and all it’s components can be helpful.
Phoenix