T2 Expansion is already a 'live' project. Part of the additional apron space (a huge bit of concrete really!) is just about active (far NW of the airport) and there are 2 new 'drive through' stands at T1 for 737 size a/c.
For the future, T2 will be extended for a huge amount of additonal capacity; T3 can be extended relatively easily eastwards and will be eventually. The big challenge however is what to do with T1 - out of date, past its sell by date etc - a real mish mash. If I were the airport company I would be planning its replacement ie demolition and rebuild in a way that solves some of the operational issues the present structures create.
My guess would be for a linear terminal one, set further north than the existing and essentially in a line with T2 running towards T3. This would create a significant amount of additional apron and airfield space. It would also create the space for a parallel remote pier offering further capacity upgrades.
As for a T4... not for a couple of decades but the location is frankly obvious viewed from the air. It is the site of the current Aviation Viewing Park. This could be linked separately and directly from J6 of the M56 as opposed to J5 for T123.
Runway 3 will be in this vicinity running parallel to the M56 but probably not for 20-30 years.