The east end of platform 4 is certainly lower than the west end. A couple of weeks ago I witnessed a poor chap underestimate the step down from a class 319 and hit the deck pretty hard. He certainly wasn't a person with reduced mobility (at least not before the fall). Anyway, he was lucky not to fall down the gap between the train and platform too. Quite astonishing that a busy station like this has such a hazard.
Unfortunately, modifying platform heights and offsets just isn't as simple as it might seem. This has probably ended up in the 'too hard/expensive pile' at NR. If you lift the platform edge the cross fall gradient can easily become too steep. If you raise the level at the back of the platform to compensate, it starts to affect building thresholds and surface water drainage. Raising the height of the platform riser wall (the bit that supports the copers) can also be problematic with certain types of construction, especially if they're in poor condition. Lowering track over the arches is unlikely to be feasible due to limited ballast depth, and besides, the proximity to S&C at both ends of Oxford Rd would probably turn a track lowering scheme into a complex and extensive renewal project. Then you've got the problem of finding the time to close platform 4 at Oxford Road for long enough to actually do the work.
The whole lot (platforms, track, footbridge and canopies - everything except the main building) was due to be ripped out and renewed by the Northern Hub programme during a full closure of the station. I expect the station was neglected for many years waiting for a major project like that to sort out all the issues. Unfortunately there's now little sign of this happening any time soon.
I'm fairly certain the platforms themselves at Oxford Road aren't listed, only the laminated timber main building and canopies. Even if the platforms were listed, passenger safety would always take priority over a listing, and a sympathetic modification could be designed to satisfy the planners.