at this rate they will need to have a 4th base aircraft
Very unlikely. From what I hear BFS-Barcelona is still doing pretty badly and loads on BFS-Paris are absolutely diabolical so if any new routes are launched they will be to replace Barcelona and Paris. Aer Lingus have concentrated their 2009 expansion on Gatwick. I don't think theres going to be any new aircraft for any of their existing bases until 2010 at the very earliest.