How to sleep train a 3-4 month old if he cosleep with us?

Any advice? Or do I need to put him in another room? He will start sleeping ard 10pm, then wake up every 2 - 3 hours in the motn to feed(e.g 12am, 3am, 6am, 9am). I will breastfeed him to sleep then put him back in his bassinet. Sometimes he poops and wakes up.