Not all 4 year olds will play with their poop but it's normal for some 4 year olds and even older kids to play with their poop and it will eventually stop.
But you should tell your 4 year old to stop playing with their poop and let them know that it's not a good thing to do and if you have to you should spank them if they don't stop.
My 4 year old nephew used to play with his poop and then smear it on the walls and he would do it when he was 2 years old and 3 years old as well when he was in diapers.
He was finally potty trained near 5 years old and would take poop out of his diaper and smear it on walls.
I finally started spanking him when he did that at my house when I was babysitting him and it eventually stopped.
Thankfully he got potty trained soon after though so we had no issues with him playing with his poop.