I have a 7.1 cf chest freezer that fits 4 on the floor and 2 on the hump with a collar. It will depend on your brand of chest freezer as well as whether you have ball or pin lock kegs. The thread day_trippr linked is your best bet to find out if 4 will fit or not.
I'm looking at a Haier 7.1, I am currently using a Haier cu. ft. And it has been great. It hold 2 with ease. I just want to sell it and add to more kegs. What brands are you using that fit 4?