According to Wikipedia (http://en.wikipedia.org/wiki/Gallon) a U.S. Liquid Gallon is 128 fluid ounces.

This means that you would need SIX 20 fluid ounce bottles, PLUS another 8 fluid ounces, to make one U.S. Liquid Gallon. (128/20 = 6 remainder 8)

If you only have FOUR 20 fluid ounce bottles, you will have 80 fluid ounces (4*20=80), which is not enough to make a U.S. Liquid Gallon.

If you are talking about some other kind of gallon (such as a U.S. Dry Gallon or an Imperial (UK) Gallon) then this might not apply. However, I know of no "gallon" which is equal to 80 oz.

Hope this helps.

Gabriel