Quiaria,
I have a 4 year old and a 1 year old. My suggestion to you... Remember always, you are the mom, you are the adult. Stand firm with what you decide. Tell your 4 year old he/she is a big boy now and he's old enough to sleep in a big boy bed by himself. Reassure him with love, but stand firm. He will scream and fuss, don't let him see you get upset and continue to put him back in his bed. As a reward for sleeping in his own bed, set up a marble jar or sticker chart and when he sleeps through the night in his bed he gets a marble/sticker. When he gets 3, give him a special reward or treat. Then move it up to 5, then 8, then 10 and so on. You can also make the rewards a little bigger as the number gets bigger. This works for other things too. Hope this helps. It worked with our 3 year old who used to climb into our bed 2-3 times a night.
Good luck. Happy sleeping