If a 4 year old outgrows a nut allergy confirmed by bloodwork, can it come back later?

Possible but rare. I believe you said that your child has outgrown his/her nut allergy and has negative blood test for allergy. The proof that you have outgrown any allergy is if you eat it again. If your child has not eaten nuts safely then you should see an allergist that will do an oral challenge with you. After negative challenge it is recommended that they eat it regularly in order to avoid it coming back.
Judicious approach. Blood work may not be the final answer. I would avoid nuts for the present.