4 weeks is aweful. spitting up alot of a sign he either has reflux or cannot digest the type of food he is getting or is too full.
baby cereal has little nutrition. all your baby needs is breastmilk (or formula if thats what you give) NOTHING else till your baby is at least 4 months old. most say around 6 months.
if your baby is not tolerating the bottled milk, change the milk first.