For 4 days, a 4000 cubic inch pack should do it, especially if you are buddying up with a couple people. It is tight, but I use a deuter 65+10 for 7 day outings without sharing equipment. I have Badlands, Eberlestock, and Sitka packs and like my Deuter the best for pack trips. As stated above, the Eberlestock is the best if you want to pack a lot of weight, but I do not like it except for heavy packing.
Backpacking is an adventure. When I first started, I packed between the low 70 lb. and the low 90 lb. range. The Eberlestock rocked for those trips. Now I am down to the low 50's for 7 days. My packs have shrunk accordingly and I make twi trips out for deer with the Deuter and get the Eberlestock J107 for the second trip on elk. I am going to get one of the new KUIU packs when they come out in a month or two. The carbon fiber frame is only a few onces and they have interchangeable bags of different sizes.