My baby is 4 months. Until now, has been exclusively breastfed. How best do I introduce formula? Will the tummy hurt, should I start with the newborn Formular or that suitable for 4 months? Will baby get constipated? Do I need to give gas drops? How many MLS/ Oz formula should I give every 3 hours?

Reason I want to supplement is because there is no way I can keep up with his appetite. He won't stop crying until he gets at least 150mls of breastmilk ( this after he has been on my breast and drunk God knows how much) and he will still want to snack an hour later if not less. I pump and breastfeed. And not enough time goes between his feeds to enable me to build up enough milk to pump. So much pressure so am going with fed is best. Do I still give him 150mls formula or less/more?