I have not stacked a cake before... was wondering, do you use a cake circle and a support dowel even if you are only having two layers to your cake? 
Or is that only for larger cakes
(bottom and top are each a little over 4" tall, bottom is a 8" round, top is a 6" round)
TIA
destini 
I would do it, just to be on the safe side. Too much time and effort goes into decorating a cake to pass by these construction steps. It's definatly worth it to take the few minutes it takes to assemble properly and keep your mind at ease! I say always use boards and dowels! Good luck with your cake!
Just to make sure what you mean...
You have two tiers, each with two layers, correct?
In other words, you have two 8" rounds, stuck together with some sort of filling, and two 6" rounds, also stuck together with some sort of filling.
If you are stacking the 6" tier on top of the 8" tier, yes, I would recommend that you dowel and use a cake board between the 6" and the 8". You might be able to avoid it, if you won't be storing the cake for long or transporting, but my philosophy is better safe than sorry.
If made of cardboard, the cake board should be covered with some sort of non-absorbent layer, such as cling wrap, to keep the board from getting soggy and collapsing.
The dowel and board set-up not only prevents disaster from collapsing cakes, it also makes serving a lot easier.
HTH.
Red Panda
Thanks everyone for your quick answers!!
And, yes, I have two Betty Crocker "starlight" yellow (8") filled between with choc. peanut butter fudge frosting (from CC recipes - thank you mehganb) and two 6" chocolate mayo cakes also filled with choc. pb fudge frosting - really yummy, by the way!!
Ok, off to stick, stack and slap on some more bc. 
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