Collectively, there have been/are taking place quite a few big infrastructure investments across the route in the last few years, such as King's Lynn platform extensions, the platform at Stevenage, the new platform and associated shuffle at Leeds, Werrington, King's Cross ect.
The Dec 21 timetable rewrite basically starts from scratch with everything from Kings Cross upwards. Thameslink has to exit and leave the core at x time, the remodelling of the throat doesn't affect the Thameslink timetable inside the core, only the timetable up the ECML. The timetable on the Liverpool Street - Cambridge runs and that part of the GEML is also being rewritten in conjunction.
As 59 pointed out, it's actually 2 extra paths in each direction every hour* unlocked by a combination of Werrington and Kings Cross. These come from the speed into and out of the throat, and the removal of crossing moves of container trains from the Spalding line across the ECML to Ely.
The four tracking is extra I think, and even then probably won't impact passenger services that much. That's been talked about for years, but still not sure anything has been approved. The ECTS rollout again is extra, but that's another five years away from completion at least.
* The two extra paths. Ofc, this is rolling stock and demand depending, but somewhere in this thread there was the proposed LNER timetable post improvements. I think 0.5 extra to Leeds and Newcastle was talked about, but idk about the other. It's likely that if Grand Central/Hull Trains/East Coast Trains fold, these services will be absorbed into LNER in some way and will also result in a timetable rejig.
