3-4 month old can often regurgitate their food/milk without effort.
.It is a rather bothersome sight for parents/caretaker to witness. Nevertheless, as long baby is not uncofmortable, still eating and gaining wt, then i would not worry much. Don't over feed and thickening the milk with rice cereal etc..May reduce this a bit. Also, burping baby to reduce large air bubble in tummy. Good luck.