Hi whengreg,
This would be a “basic” 2 layer memory foam mattress.
4 lb memory foam is in the mid range of density/quality (5 lbs and higher is high quality and less than 4 lbs is lower quality). There are many people who prefer the feel of lower density memory foams because they are often faster responding and not as temperature sensitive and “feel” softer at least initially although they will not be as durable as higher quality memory foam and I would be cautious with a mattress with 4" of 4 lb memory foam if you are above an average weight range.
1.8 lb polyfoam is good quality and well suited as the base layer for a mattress. Some better quality or higher budget mattresses will use higher density while some lower budget mattresses will use 1.5 lb density. The base layer of a mattress is not usually the “weak link” of a mattress because a mattress usually softens and breaks down from the top layers down.
An example of a major brand mattress that uses similar quality materials (4" of memory foam in the 4 lb range although the base layer is 1.5 lbs which is lower quality) is the Sealy Optimum Radiance … which is not good value.
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An example of a mattress that is also similar (4" memory foam over a polyfoam base layer) but uses higher quality materials (2" of 4 lb and 2" of 5 lb memory foam and a similar quality base layer) and which provides the ability to customize or rearrange or exchange the base layers and memory foam layers is the Rocky Mountain CoolComfort 10" here.
Another example that also uses 4 lb and 5 lb memory foam over a 2.0 lb base layer (higher quality materials) is the Novosbed Aria here.
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Another example is the Dreamfoam Supreme Gel here which also has a 1.5 lb base layer and 4" of memory foam on top (including a gel memory foam top layer)
These are a few examples with somewhat similar designs (4" of memory foam over a polyfoam base layer) that you can use for a “value reference” from the list of online memory foam mattresses in post #12 here.
Knowing the density/quality of the materials gives you the ability to make meaningful quality and value comparisons with other memory foam mattresses you are interested in either locally or online.
Thanks for listing the specs 
Phoenix