I should think that the current terminals will be able to cope for some time yet before talk of a new terminal does the rounds. Although T1 and T3 are nearing full capacity at peak times, T2 is still extremely quiet after midday so plenty of room for additional capacity there.
Also, I think that the terminals will undergo major expansion before a T4 is considered. There is plenty of scope to do this. T1 is in the process of this as I speak. With the new security search area being constructed above ground level checkin and the new retail area above the current airside lounge due to open shortly, some pressure should be released on its cracking seems. There's plenty of room to the east to extend T3. I read somewhere that eventually it will be extended all the way as far as the airport pub. Was this just hearsay? And there's plenty of room to extend T2 as well once it reaches full capacity. The plans were to add onto the exisiting checkin hall and departure lounge, as well as extending the pier from gate 215 down towards staff west. This particular project was due to have started by now but all has gone quiet. Anyone heard anything?