Welcome to Flyertalk, JenE

From ba.com:

'TERMINAL 1:
From Terminal 1 to Terminal 1 - 60 mins
From Terminal 1 to Terminal 2 - 75 mins
From Terminal 1 to Terminal 3 - 75 mins
From Terminal 1 to Terminal 4 - 75 mins

TERMINAL 2
From Terminal 2 to Terminal 1 - 75 mins
From Terminal 2 to Terminal 2 - 60 mins
From Terminal 2 to Terminal 3 - 90 mins
From Terminal 2 to Terminal 4 - 90 mins

TERMINAL 3
From Terminal 3 to Terminal 1 - 75 mins
From Terminal 3 to Terminal 2 - 90 mins
From Terminal 3 to Terminal 3 - 60 mins
From Terminal 3 to Terminal 4 - 90 mins

TERMINAL 4
From Terminal 4 to Terminal 1 - 75 mins
From Terminal 4 to Terminal 2 - 90 mins
From Terminal 4 to Terminal 3 - 90 mins
From Terminal 4 to Terminal 4 - 60 mins

Please note: Connections at any terminal to/from the Republic of Ireland requires a minimum connecting time of 90mins.'

If I were booking a T4 to T1 transfer I'd leave more than 90mins but T4 isn't heaving at that time so I'd have thought you should be fine. Those connect through LHR more than I should be of better use!

As for the overbooking your best bet is to input the details of your flights into this availability tool. http://flyaow.com/classcwt.htm

J, C, D and I represent the numbers in Club Europe. If it is high numbers for the most part you should be fine with getting onto the next flight. CE is just a convertible seat so someone with the key should be able to move it but I'm not sure how flexible they are with doing this for last minute changes. My guess is you should be fine.