Can I stack 4 layers of cake without using the cakeboard in the middle?  It will be multi color and need the presentation of being able to cut the cake and see all the layer colors.  How can I stabilize it?

Yup you sure can.  I have made a 4 layer cake before and didn't use any internal support.  I just made sure the layers were level when stacking.  Of course if you get concerned about stability you can stick a dowel through the center of the cake, but as long as you keep things level you most likely won't need one.

The usual 'rule of thumb' is to put a board between each 4" or 5" tall.  It does depend on how tall each layer is, the size of it and how far you need to transport it.  If they are only 6" round, you might have trouble moving it much.  Stack up some styro or pans - 6" rounds 8" to 9" tall (four 2" layers with filling!) and you will see how much of a problem it could be.  A center dowel needs to be a b it 'stout' - not thin in order for it to help stablize a cake - and again, it depends on the size of the creation - rounds or other shape?

Thanks for the replies, the layers will be 8 inch diameter, and usually close to 2 inches tall each.

All of my cakes are 4 layers. Typically with buttercream they stand right over 5 inches tall, sometimes 6 if I'm crazy generous with the filling. Lol. You should be perfectly fine!