Eh, figure out how much sleep he gets. If you get rid of the nap then he'll be loud and not listen while his sister is trying to sleep. And that will KILL you and the nanny. So keep the nap.
It might be that after the surgery when he was recuperating you (naturally and necessarily) were more forgiving because he was in pain, had just had major surgery, etc. so you gave in more easily, didn't have as hard rules, etc.
Is this possible? In which case, then it might be that he's so used to that, and he's now expecting NO boundaries and limitations. So then getting a little bit of parenting help might help you to set limits and stand by them.
Alternatively, it might be that the other 4 year olds have grown up and are more able to regulate their behavior (not pushing, shoving, hitting, leaning on kids, listening, etc) but he isn't - in which case it might be related to the surgery and your response to it OR it might be that there is a bigger issue and you should figure it out and start teaching him how to regulate before kindergarten.
So yes, ask your pediatrician for a list of parenting coaches/therapists. One place you can also look at it Jonah Green Associates
https://www.childandfamilymentalhealth.com/