I inadvertently paid an employee's health insurance for four months after leaving my employ. Can I collect these premiums from the employee?

Yes, you can. Of course, if the employee does not want to repay them, you would have to sue him or her to recover them, in which case you will need to weight the cost, inconvenience, and disruption of the lawsuit vs. the amount you could recover. You should also, before going ahead, review any and all correspondence with the employee, leading up to, around, and after his or her termination; if you wrote or said anything that suggested that you would pay health for a period of time (such as part of severance), it may be difficult to win if the matter ends up going to court.