4 aligned foot fractures (so no surgery). doctor removed the fiberglass cast for the 3rd-week xray, then prescribed the walking boot. is it too early?

Boots are great: In my experience, such fractures seem to have a good bond at 3 wks. Walking boots actually immobilize quite sufficiently. Just don't over do it, as healing is still occurring.

Several factors: Yes or no depending on the bone fractured, the alignment of the bone(displaced or not), the radiographic signs of healing, how much and how long weight bearing.

No: Treating Ortho checked follow up XR and examined you hence is in better position to give sound advice