I'm torting a cake 4 X 7 inches in size.

I have it now at 3 1/2 inches high with another 1/2 inch to go. I just realized maybe in need to be putting a cardboard in there somewhere????

Am I okay at 4 inches or should I insert cardboard at half way (2 inches)?

I am building a purse and it will be tapered toward the top.

It should be okay. I recently did a 6" high cake in 4 layers with no support, and it was pretty solid.

+2 4" is fine