I had this issue when the Prime 4 was first installed at work. I’d had it for a few weeks and then moved it where I started getting the mixer not being online and play/cue not working. I think it happened around 10 times but after a few weeks it has stopped and hasn’t been seen since.
I’m always keeping my eye out for it though but Denon DJ was great and referred me to a dealer if things persisted. Luckily I didn’t need that.