That sucks.. For 4k and a ruined credit score I would probably just go ahead and knock on their door tbh. Or keep trying with the utility company like someone else said. Good luck, hope you find a way of fixing it.

Have you tried reaching to the new tenants? Could be an honest mistake, maybe you could get them to pay. I remember when I was younger I moved to a new place. For some reason I thought gas was included in the rent. For a few months I used it without any issue so I never realized it actually wasn't. One day the manager came with a huge bill that I needed to pay. I explained and apologized, called the gas company to put it in my name and paid the manager back. I mean, you could give it a try.. Otherwise it seems like you are stuck with the bill