Hi alexb80,
The PLB version 2 models used to have an inch of very firm latex on the bottom (firmer than their “normal” firmest core which is 44 ILD). When I first asked them about this they told me that they were “in the range” of about 50 ILD (which was listed in the specs posted here by another member as well) and then on a later call they told me that they wouldn’t be so specific about the ILD and it was probably “safer” to list them as extra firm so I edited the same post. You can see one of the posts that I didn’t edit here.
I didn’t ask them this but they may have “targeted” for 44 ILD but ended up being a little firmer on average or they may actually have made them this firm to act as a ultra firm stabilization layer to replace the polyfoam that they were using previously.
This is another one of those “it depends” answers. It would probably make less of a difference than changing the comfort layers for most people but it would depend on how deeply you sink into the mattress and how much of the “feel” of the core you can feel through the top 4". Higher weights or side sleeping positions or people who were more sensitive would probably feel it more than lighter body weights, “flatter” sleeping positions, or people who were less sensitive.
Phoenix