I'll be doing a very small "potty" cake later. So for the tank - I made
2 - 8 X 8" squares and cut them in half to get 4 - 4 X 8" rectangles. Do I just make it a 4 layer or do I need to seperate and secure/support it? I just need some height.
TIA

You need some support.

I think it would be ok with 4 layers stacked. If you want the added security you could put a couple of dowel rods in for stability.

Basic rule is to use a support system for every 4" of cake height.