Hi re-vogel,
Density is the single biggest factor in the durability of a foam material but not the only one (see post #4 here and the posts it links to for more about durability) so I would say it would be closely comparable to other memory foams that are “about” 4 lb density but memory foams in a 5 lb range would generally be more durable than memory foams that were in the 3.7 - 4.2 lb range so with higher weight ranges (low 200’s or higher) I would tend to minimize the use of 4 lb memory foams in a mattress (see the guidelines here) while for average or lower weight ranges they would normally be fine.
Phoenix