Pretty standard around here, I have 5 friends in the area with 4.00" floors in thier shops, they hold up just fine (actually 4.5" is where mine will be). Bed of 12.00" compacted crushed rock, .006 vapor barrier, 2.25" thick foam sheets, 6"x 6" steel wire grid mesh on top of the foam, 5/8" PEX tubing for the radiant heat, a grid of 1/2" rebar on 12.00" centers above the tubing. 5 sack mix concrete and It'll be PLENTY strong for home shop use.