what size diapers is you LO in?

Daughter will be 7 months in a few days and she has been having blow outs for the last week. She is currently in size 3 diapers and she's tiny for her age! But now I think she needs to move to size 4. I hate to return the box of threes that I just got and get 4s but I'm thinking maybe that's the cause for her daily blowouts! I would just get a small box of the 4s to try out but they only carry the big boxes in size 4 pamper cruisers!