I'm going to try making one of these next week with a 9 inch pan, for those who have made them, is 4 layers of cake enough for a proper height?  And if 4 layers is enough, should I put a board between the two layers?  Thanks for any tips!

If your layers are between 3/4 and 1 & 1/2 inches you should be fine without additional boards. I would like a 9" square present cake to be about 4 inches high give or take.

If you are making a real tall big boxy present cake 8 inches tall then you need another board about every four inches of height. But I mean a five inch tall cake could stand ok without a board.

I agree with -K8memphis (and, believe me, she has lots of experience!). I use a board every 4 inches (every two layer, if I'm lucky!), not only for the support, but for the serving size.

Thanks...so I'll do 4 layers, and just stick some hollow plastic dowels in them, to prevent sliding then---learned my lesson with sliding the hard way on a delivery once!  I can laugh now, but then it was devastating :)

Appreciate the help.