The answer is : (thx Hao Wu)
^\d{4,5}+(,\d{4,5}+)*+$
The + after \d{4,5} makes \d{4,5} possessive, I keep it to avoid backtracking to 4 numbers if 5 matched. The + after (,\d{4,5}+)* makes the group possessive so we keep progressing.