I want to stock up on some diapers using this deal (http://hip2save.com/2014/07/02/new-video-the-best-upcoming-target-deals-76-712-hot-deals-on-diapers-more/) (4 Pampers boxes for about $14 each) but I'm wondering what sizes I should get?

From gifts/hand-me-downs of open boxes that people haven't used or have grown out of I have the following already:
315 Newborns (I know this is a ton, but people just keep giving them to me)
260 Size 1s
145 Size 2s

I'm thinking 2 boxes of size 3s and 2 boxes of ?? I'll definitely keep the receipt so I can exchange them if he ends up not using them. Thanks!