I am doing a 4 tiered wedding cake (14" and 10" carrot cake, 8" dummy cake with pearls wrapped around it, and 6" carrot cake on top). Can I dowel all 4 tiers, or at least three and add the top 6" at the reception?

Yes, you can put the top cake on at the reception. And your correct, you won't need to dowel the Styrofoam dummy layer.....the 6" will be fine on top of it. The only thing you must do is secure the 6" onto the dummy well with a dollop of frosting and your border.

Sorry, I should have specified heights in my original post, as well as specifying the question of a center dowel :( 

I normally wouldn't put a center dowel in cakes that are only 2 tiers tall. If the 3rd tier is a tall dummy, will that be okay to transport without putting in a center dowel? Would it be safe to stack all 4 tiers and inserting a center dowel, or is it not possible to put a center dowel in a Styrofoam dummy?

You do not need a center dowel for your cake.  Many of us have transported 2 and 3 tier cakes w/o using one.  You just need to be careful driving as one should be delivering *any* tiered cake.

Yes, a center dowel can be driven through the styo tier just as one would a cake - maybe need a bit more push to get it through but as I said above, you really don't need one at all. 

I would put a dowel/stake in the styro to hold it onto the cake below it. The dowel doesn't have to go all the way through ....it's just that those light styro's can be bouncy in cars. Than put 6" on at location.

If you want to drive with all 4 cakes stacked then dowel though the whole thing.

The ONLY cakes I center dowel are Styrofoam dummies.

The icing will hold the styro in place.

Thank you so much for everyone's help :)