No foundation is just a bad decision, and here's why. First, if you leave somewhere where it's gets to 32 degrees or below, the ground freezes and this will cause the slab to (what is called) "heave". That means it it actually rises. That will cause the slab to crack (even with steel rebars in it) and displace up and down. Second a straight concrete slab (at 4" thick) cannot support the load of the framing you want to place on it (as well as the weight of an automobile). This will cause the concrete to break. If you want to frame with wood, you need to identify what the "frost line" depth is in whatever region you live. Here in Maryland for example it's 30". That means your footings need to be dug to 30" depth below the ground level. From that point you need to pour a minimum of 8" thick by 16" wide to support the structure. You have a choice of either pouring the footing solid up to 2"-3" below the grade/ground outside and lay 1 course of cinder block, or just pour 8" thick and bring it up above grade with 4-5 courses of cinder block, or you can form it with boards and pour the whole thing footings and slab in one pour (called a monolithic pour).