Accidentally fed my 9 months old baby formula that was sitting out for 4hours in an air conditioned room. what are the risks?

Minimal: Formula that is left at room temperature for that duration after a baby has started feeding, may have a chance of germ build up.This might produce some indigestion and loose stools.This is less likely if it was just left out but not as a feed.

Throw it out: Simple answer is to throw it out. Your formula container will generally tell you to use formula within one hour after reconstitution. Risks would be the consumption of spoiled milk leading to bacterial enteritis with vomiting, diarrhea, possible dehydration which may need IV fluids or hospitalization. As i said, just throw it out--the risks just aren't worth it.