They are only assuming she's 4. How could they know for sure?
If she gets a good medical exam by people who know what they are doing they can get pretty darn close. It involves also doing a dental exam and possibly a dental x-ray. A forensic dentist can tell a lot about a person, including age, by examining teeth and xrays.
My heart is going out to all those who are missing a child tonight, and that poor baby girl. There is a good chance she isn't actually missing, but was sold etc by parents.
ETA: now it is thought that she is 5 or 6. And there are no allegations that the people who had her were mistreating her in any way. She was dirty and living in poverty, but so are all the other kids in her community. The people who had her might be telling the truth, that this child's mother didn't want her so they took her in. If they are Roma, then it's not like they could or would go to the authorities for a legal adoption.