One thing that you might do is pick up a cheap 12" one from Goodwill or borrow from a friend when she is ready to transition to a pedal bike. I found that after the balance bike, a 12" was good for a month or two of transition time where they could have their feet flat, etc. After that, we moved up to a 'good' 16" bike (from CL ).
DS2 used a balance bike until age 4 and then moved to a 12 in bike for a few months and then onto a 16 in Trek. He will move to a 20 in this year at 6 yrs. Also, keep in mind that different bikes fit differently. The Trek bikes, for example, tend to have a lower seat, so they can fit on them sooner which is good for my tiny kids.
Mama to three boys ('03, '05, '07)