Which size would you use (4 or 5)?

My daughter is currently in the size 4 in the chest, with extra length added on top (she's petite, but long-waisted), but a smaller size on pants (size 2) In the case of tops/dresses, most only go through size 4, then buy the next package for size 5+. So…if she's currently a 4, do I stick with the smaller size for now? Or is the 5 not *too* much bigger, that I might be able to get away with that one? I simply hate to invest so much when it's the last size (then again, I have no idea when she'll grow again)