I would definitely get the four cubic foot bag. Most likely you won't have much left over....if any. How deep are your beds?

Let's say they are 12 inches deep. That would mean your 4'x4' bed would hold 16 cubic feet of soil. If you use roughly 1/3 compost, 1/3 peat moss, 1/3 vermiculite that would come out to just over 5 cubic feet of vermiculite.

If you have beds that are 6" high, you would need about 2 1/2 cubic feet. Not to mention you'll need more of your soil mixture from time to time to top off your beds.

I would suggest getting the 4 cubic foot bag and filling the remainder of the bed with a combination of compost and peat moss.
Kim

Here is a link that might be useful: Kim's Garden