S.T.
this is a kid thing, not a YOUR kid thing.
honestly, blaming one's kid's ever-evolving behavior on 'mimicking' is a subtle cop-out. i say that gently. we all figure things out as we go. but think about it. 'my kid has always been just fine, but he changes when he's around other people! oh noes!'
your kid will always have other people influencing him. that's life. your job isn't to prevent him from being influenced by anyone else (a lot of those influences will be positive) but to make sure that a) you're the main influence and b) that you're working with him on weeding out and becoming discriminatory about the behaviors he chooses to mimic.
please note, i'm framing this response for an adult, not a 4 year old. he doesn't understand the very natural processes he's experiencing. he's only 4. this isn't a one-and-done fix. you don't just tweak something and presto! your 'sweet well behaved boy is back.'
he's still there. he's learning and growing, and that means constant, ongoing change. some of the changes aren't for the better, so you gently and patiently redirect them.
you don't just slap an 'other kids are causing this' label on it and think you can 'fix' it.
this is parenting. it lasts for a couple of decades.
khairete
S.