Foam Density for bench seats

[Chevy Eleven]

Hello-

I am looking to order some 4 inch foam for my Chevy II and I have questions... like what does 4128 mean?

Here is what I am looking at http://www.rochfordsupply.com/shop/F...128/index.html the 3 or 4 inch stuff

I want to have a comfortable seat for long trips, and can not find what was used origional. Also is the 4 inch to thick for a factory bench seat?? Thanks for the help!