When using stacked construction - if my bottom layer is 8 inches, would you use 6 inches for the top, or 4 inches?

You can do whatever you want. A 4inch is pretty small. What is the cake for????

I think it's a matter of personal opinion. I like to have 4" difference but I have also seen some very beautiful cakes with less.

To help me decide what to bake, I will stack the pans on top of each other to get a feel what the sizes will look like together.

HTH

It all depends on the look you're trying to achieve. Either one would work.

Try taking your cake pans and stacking them to get an idea as to how each size would look.

If you're making a cake with 3 or more tiers, remember to be consistent in your size differences between layers. For example, stacking cakes that are 14, 12, 10, and 8 inches would look more even than if you used 14, 10, 9, and 6 inch cakes.

Hope that helps!