How do I make a horizontal elevated cement or concrete surface supported by cement blocks on 3 sides?

First, plan on 6" thick and not 4". Get some 3/4" construction grade plywood and build a fully supported layer even with the top of the blocks. Use 2x4s and concrete blocks under the plywood to allow temporary support every 12" using shims to hold the plywood level and flat. Use 2x8 side perimeter boards supported level from below and braced to hold the weight. The 2x8s will be positioned so that 6" is the slab depth and the 1-1/4" remainder overlaps the the outside of the blocks all the way around. On the open side, firmly support under the plywood to the 2x8 to hold the weight. You will use concrete steel mesh to support the concrete and you'll want to make sure that you lift it as you pour so that it is 3" off the bottom, (dead center). As for heat, burn your coals on a low grate and not directly on the concrete.