I want to buy a set of round 6, 8, 10, 12" pans. For 2" pans you can only bake 1 layer at a time...do they make 4" pan set?

No, they do make 3" deep pans, which if you torte and fill you will have a 4" high cake. I prefer to use (2) 2" deep pans for each size, I find cakes bake better in the 2". I fill one pan 2/3 with batter and the other just over 1/3. I level both cakes, but only torte the taller one. My finished cake is 3 layers of cake, 2 layers of filling. I find that to be a great balance of cake to filling.

Agree. Those 4" deep pans are primarily for fruitcakes. Cakes bake more evenly in a 2" pan.