My baby is 4 weeks old. when she has her bottle with formula she brings up a lot of milk. what could be the cause?

The basics: Infants that age can swallow air & create a stomach full that triggers spit up.They can also have difficulty keeping the feed down since the stomach entrance is a fold in a tube rather than a valve. Re-flux or backwash is possible in any infant & tends to amplify as they get older, peaking at 6-10 weeks in some. Overfeeding a "barracuda" feeder is also a possibility.