Update on the situation:
Bike still won't rev above 4000RPM. I've tried slowly getting up above 4000, and also gunning the engine and either way it gets to 4000 and that's the end. If I don't throttle down slowly the engine stalls on me. I've done this while in neutral on the centre stand, and while attempting to ride the bike.
I've adjusted/cleaned the spark plugs and they are firing fine.
I've also noticed that the engine seems to be at a lower pitch than it used to be. It wasn't a high pitch whine before, but it definitely seems to be a lower rumble to it.
I'm wondering if there is an issue with the carburetor? Or perhaps an issue with the amount of oil (all this started after I changed the oil)? I've also read that maybe airflow could be an issue, would a dirty air filter cause this?