Hi Philippe; Welcome to Permies!
To
answer your question, 4" is the average layer of insulating cob under a mass.
Normally, to mix cob to be insulated, you would want to add cut pieces of
straw / hay / horse
poop to mix with your cob.
Generally perlite is not used under the mass. Straw is much cheaper than perlite. No reason you can't though.
Using bricks and cement board to elevate your mass off of the floor is also a common practice.