We're in a similar boat with my 8 1/2 mo old. After two older kids, I'd say feed him and put him back in his crib and walk away. He will learn that this is not "wake up and play" time, it's just refill the tummy and go back to sleep time.
At about this stage I hung a few toys in our son's crib. One was a mirror and the others were using those plastic links, to hang something on the crib rail that would be about eye/arm level for him. I also left a pacifier in there for him to grab/chomp on.
Very soon he learned that "his friends" as I called them, were there for him to explore. When he'd go down for a nap, I'd put him down drowsy, but awake. Sometimes he'd fuss a bit, but soon the colors of his toys hanging on the side rails would catch his attention. He'd bat at them and talk to them.
Another mom also told me that when she was doing this "training" to get them used to their crib and learn how to play there a bit. She'd put her son down about 10 mins before his naptime. She'd walk around the house, singing, putting away laundry, walking in and out of his room so the baby could see her, but she wouldn't pick him up. She'd just smile, say hello and keep going about her business. Her son soon learned that mom is still there, and that my crib is an ok place to hang out and play until I get tired.
So back to your son. Good job on the sleep training. Now see if you can start teaching him to "self soothe". When he wakes at 4am, feed him with the lights low or use a nightlight, minimal talking and no playing. Feed him, rock him and put him back down and walk out. He may fuss a bit. That's ok. He'll learn that it's not time to wake up yet and (hopefully) go back to sleep.
It's a process. So rock him or sit with him until he's sleepy. DON'T PLAY or stimulate him. You'll just teach him that he's being rewarded to wake up "at night". He'll get it and you'll get some sleep.
Good luck Mama!