wash can work, depends on the milk and make. But overall, if salting and hitting your pH markers for rennet, drain, and mill/salt are not doing it, AND if a cold crash isn't doing it either, then you have some oddity in the bacterial payload. Perhaps your native LAB are having a synergistic reaction with your starter to rapidly accelerate pH curve or do a too-low terminal. IMHO, if nothing else is working at all, including pasteurization, then call it a quirk of the milk and flora and live with it. a 4.9 finish for cheddar is not the end of the world if you hit the drain and salt pH.
edit: I would try different starters before doing that, though.