My smallest is 4mo, too, and about the same size. The larges fit her great. The elastic on the AIOs is much tighter that other SB products- the mdium pockets gape on her legs, but the large AIOs are fine. I have both snap and aplix, and both fit her. I actually just traded all of my mediums for larges with another mama because the large AIOs can fit all 3 of my kids. Just FYI, if that helps at all.
Jenn, mom to Trent, Gavin, Abbie, Carolyn, Bethany, and Ariel! Alice.com is awesome,Swagbucks really works, and earn rebates with Ebates!