I only see 4 inch columns for sell. My decorated cakes average 4 1/4 inches high once they are filled and decorated. How would I use this system? In the videos I watched they were cutting the columns to height. Can someone suggest a place where I can find taller columns? Thank you.

At the bottom of this page are some 9" columns:

http://www.oasisupply.com/Products/Products.asp?CategoryID=303

Yup..You will have to cut the 9 inch ones.

Where are you looking to purchase. THey have 4 inch and 9 inch I think at Oasis cake supply. Be sure you have a saw or something to cut them with because they are sturdy. Can you level your cakes just a hair shorter and stick with the 4 inches of height. I've had mine just slightly over 4 inches and it wasn't a problem for me but you want the legs to go all the way to the cake board.

I've only used SPS once so I'm no expert, but you can buy 9" columns that you cut to size: http://www.globalsugarart.com/product.php?id=21161&name=Grecian%20Column%20Multi-piece%209%20inch%20%2812%20pack%29%20by%20Bakery%20Crafts
But that is a lot of wasted product for 1/4 inch... it might be worth your while to try and make your cakes 1/4 inch shorter but trimming the cake a little or add a little less filling?

If I was to trim the two cakes how much should I take off? The cakes will be frosted in butter cream and covered in fondant. They want 29.00 for one 9" pack with shipping.

Take 1/8 inch off each layer and that will remove the extra 1/4 inch overall.