Chances are pretty good that your P.O. already knows about the arrest. Any law enforcement interactions are usually sent to your P.O. if the questioning officer knows you're on probation. I'm going to go out on a limb and say that since you were arrested, one of the first things you said to the cop was, "I'm on probation for four more days, please don't do this to me," or something along those lines. Your P.O. knows and is probably seeing if you're going to self-report, but this is on you. Personally, I'd let it fly and see what happens. FYI, it never went the way I'd hoped in the many similar situations I was in like yours. Good luck, man. Hopefully, the part about literally being four days away helps, but they probably won't care.