I have an order for my first fairly large wedding cake out of butter cream. They want 4 square layers and 4 round layers stacked directly on top of each other. I am wanting all the help I can get on the support of this cake. I guess I am just a little nervous because it is for a friend and I really want it to be outstanding and nothing to go wrong. I am also going to have to deliver it and that always makes me nervous. 
Hi, what are the sizes of the square and round cakes? So the round cake has 4 layers and so does the square one?
First you need to fill and frost them seperatly with buttercream. Then you need to dowel the bottom cake (square?) with dowels or bubble tea straws. Then the cake that goes on the top should be sitting on a board which is same size as the cake. When you insert the dowels or the straws, insert half way through. So few inches of the straws are above the cake. Then put the top cake centraly on the dowels and let it go. So the straws sinks itself nicely into the cake. So you won't mess the frosting of the top tier. Then you need to insert a centre dowel as well. You can deliver the cakes already stacked like thi. Hope it make sence. This is sugarshack's technique.
But some tend to use SPS system. Which lot of members say its safe to deliver already stacked. It all depends on what you suits you best. 
Each tier (2 layers of cake/filling to = 4") is on a cake board the same size as the cake, then iced. Once they are all iced you can start putting the tiers together. I strongly suggest the SPS. If that is totally not workable for you (time restraints) then the Wilton hidden pillars is next on the list. It is somewhat similar to the SPS. You use one plate that locks into the hidden pillar 'tubes'.
If neither of those options are possible, then plastic drink straws are next on the list. Last option (IMHO) is wooden dowels that bashini described.
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