I always feed formula milk first, and then if my LO doesn’t finish it, I use the excess to make cereal. (After 6mo, I also use water to make the cereal). If you’re just starting out, and especially at 4 months, milk should still be the mainstay of nutrition. You also don’t know if baby will be able to finish the cereal. When you start out, you should give a few spoons’ worth of cereal, 1x/day. Then gradually increase. So perhaps, give baby the 150ml of milk, then make a little bit more milk to make cereal. When you start increasing the amount of cereal you give, then decrease the volume of milk.