When stacking a 4 tier cake and all the tiers will be covered in fondant, what is the best way to stack...should I use cardboard cake circles and dowel rods or should I get those plastic plates and then the plastic rods made by Wilton? Also the cakes will not have any type of borders around the bottom....

Up to you, if you are stacking on site than it doens't matter as much but if you are travelling with it than I've heard good things about the SPS system.