I am pretty sure that if slabs are half a block that they will be like 1 iron for 4stacks of slabs.
Now, with cakes that cost 4 iron in op (i think its ten in normal) you cannot even build a bridge with that, you cannot build stuff with cakes, the only that you can build is food. With cakes you cannot build in air, and you cannot build more floor and not a wall, you can place it in the wall i get that, but there are lots of other things you cannot do.
Cakes are there to have food placed on the floor so you can just go by and eat it, which saves you inventory space.