I'm not 100% sure what constitutes "Play teasing" but I am sure your intention is to have fun and strengthen the bond between you and your grandson. Here's the thing, though, 4 year olds aren't known for their ability to differentiate between "this is a funny joke between me and my grandpa" and "this might hurt my friend's feelings."

Simple solution, stop the "game" and let your grandson know that teasing can hurt people's feelings that's why you're counting on him NOT to tease his friends (or anyone else)... not even the "play" kind. Does this mean you and your grandson can't continue to have fun times together? Of course you can! Just watch what your games with him are really about. Are they about jokingly putting him down, competing with him,  or trying to fool him in some way...? If so, think about replacing those games with interactions that will build trust and self-esteem.

That's how you develop a friendship with the boy and help him develop good friendship skills.

I hope this helps.

In friendship,
Annie