Hi 01svtL,
4 lb is a density for memory foam not a softness rating.
While all memory foam is relatively soft (with different degrees of softness) in most cases … 4 lb memory foam will be softer than higher density memory foam but there is a wide range of different types of memory foam in each density and each of them can have different properties that affect how they feel and perform (see post #9 here). A knowledgeable retailer or manufacturer that sells it is usually the most reliable source of information about how a particular memory foam may compare to others.
Some of the better options I’m aware of in the Houston area are listed in post #2 here.
I don’t know if the layers are glued (you would need to cut it off) or if there is a fire barrier around the foam (which would need to be removed) but if you have direct access to the foam layers and you could remove them it could work. I don’t know if the quality of the other foam layers you are building on would justify the time and effort though and if you get the design wrong then you would have no recourse and it could become an expensive experiment. I think that mattress surgery would be a last resort … not a plan.
You can buy foam glue at any fabric shop.
Phoenix