M is currently sized at 2 1/2. I want to get her a pair of fall/winter boots for next season. Should I go with a 4 or 5??

If their feet grows about 1/2 size every 3 months she'll technically be a size 4 in January but I would put her in a size 4 probably starting October. In which case it should last her till April? But is that not leaving enough free room? Then I'm afraid 5 might be too big?

help!