It is recommended to wait at least 2 weeks to properly evaluate the effects of switching infant formulas on a baby. At 4 weeks old, babies are still learning to digest on their own, which can come with difficulties. It is important to make sure the nipple used is the appropriate size and to burp your baby frequently to release gas. Avoid laying your baby down or changing positions too often for at least 20 minutes after feeding. Offering smaller, more frequent feedings may also help. As long as your baby is wetting and soiling their diaper, gaining weight, and overall in good health, it is best to monitor the situation. If you have any further questions, feel free to reach out.