Hi aehome,
Post #2 here and the posts it links to has some topper guidelines that may be useful.
No … some polyfoam is good quality but it depends on the density. The use of low density polyfoam is the biggest issue … not polyfoam itself. I would look for a minimum of 1.8 lbs if you are looking at conventional polyfoam and if you are looking for the best quality then HR (high resilience) polyfoam (starting at 2.5 lb density) is the best quality of all and has some of the qualities of latex (higher resilience, good compression modulus, good durability etc). Post #4 here has some sources for polyfoam of any type or quality. Local foam shops may also be a good option.
The type of material you use in a topper is based on personal preference and there is no specific reason to go with this type of combination. While it may happen to work well for you … if anything these types of complex layerings are much more difficult to predict how they may feel and perform for you. I personally would tend to focus on materials or combinations I was familiar with rather than unknown combinations.
That depends on how you define quality. Density is most closely connected to durability so a 5 lb memory foam will generally be more durable than a 4 lb memory foam but there are many people who prefer lower density because they like how it feels better (lower density is usually faster response, less durable, and less temperature sensitive although this can vary widely depending on the chemical formula that is used). You can see some of the different properties of memory foam in post #9 here. 4 lb memory foam is in the mid range of density and I usually don’t suggest going lower than this.
I think the best way to buy bedding (including a down alternative topper) is to deal with a retail outlet that is knowledgeable and can give you the benefit of their knowledge and experience about the products they carry. Most of the better ones put some good research into the products they carry and can give you the benefit of their research. Without this there are so many down alternative or fiberbed products on the market from cheap and low quality to more costly and better quality that you are really depending on the luck of the draw if you deal with someone that doesn’t have specific longer term experience with a particular product compared to others. Post #10 here about mattress pads has some good resources and post #26 here about sheets and bedding has some good links as well. You can see a few fiberbed toppers here that also have good ratings but reviews tend to be based more on the initial experience of users shortly after a purchase and may not reflect longer term durability and fiberbeds as a group tend not to be as durable as natural fibers like wool or good quality foam toppers.
Both have been in business for a long time and I wouldn’t hesitate to deal with either of them. As you pointed out … a good return policy can be important because when you are buying a topper to some degree you are going by trial and error and a mistake in terms of foam type, density, or thickness is easy to make if you haven’t tested a specific combination in person.
In general when you are buying a topper that you haven’t tried in person … the three most important parts of your choice are personal experience on the combination (or a very similar combination) you will be sleeping on (mattress and topper), the knowledge and experience of the retail store or manufacturer you are buying from who can give you better guidance based on their experience, and the return policy or tehe retailer or manufacturer you are buying from in case you make a mistake and “best guess” (which is the best you can do without personal experience on a combination) turns out not to work as well as you hoped.
Phoenix