First, and foremost, if you haven't already please tell your SO about this. I'm sure someone in a four year committed relationship would have already, but just in case you haven't, tell your SO.

I think there's a few options here; report him to the person above him, ignore him, and send an email.

If I were you I would probably send an email, but I think all the options are valid. I would say basically what you said here

• You did not flirt with him at his party

• You're not, nor have you ever been interested

• You know he's married with kids

• You're in a committed relationship

I'd also add something like "at this point I find your interactions with me creepy. It's disgusting you did this while married. Your messages uninvited. Do not message me again or I will report you." This way if he replies again, you have proof his advances are unwanted.

I'm sorry this is happening to you, and I hope things work out. Keep us posted.