Back in the 90s there was a weekday 0600 Edinburgh to Kings Cross calling Newcastle and York, scheduled to arrive 0959, I.e. (just) under 4 hours with 2 intermediate stops.
I used to commute on it quite regularly. However I also know from that experience that it wasn’t the most likely to arrive on time! Plus I accept that there are more trains running these days so such paths are more disruptive. My point though is that it can be and has been done.
At the start of electrification there were 2 up and 2 down services timed at 3h59, these were 4 of the 5 so-called Scottish Pullman services. All called at York and Newcastle only, except the single non-3h59 service.
An evening service calling at Darlington instead of York took 4hrs 08 and the northbound Flying Scotsman was also 4h08 with calls at Peterborough, York and Newcastle.
One Scottish Pullman was actually timed through to Glasgow in about 4h55 IIRC.
Timekeeping wasn't great, with only 1 min recovery margin in one direction and after just a year, the full standard amount was added back into the schedules.
Nevertheless the 3h59 timings came back after a few years, but seem to only have applied to one service each way after that. They disappeared again at privatisation but GNER brought them back briefly in the early-2000s. After one year, one was adjusted back to a normal schedule with more calling points, the other was padded out to 4 hours 2 mins.
Even in the days when a few mph over the limit was more common, absolute right time arrival was about 30%.
After that, I think the next fastest is the up only 4 hours timing of the Flying Scotsman which has applied since the 2011 'Eureka' timetable.