I don't see it as a gimmick really. If we do see a 4D cinema, I'd imagine it would most likely be in the ice tent, it really wouldn't suited to X Sector and they'd be daft to consider putting it in there due to the possibility of it being used for a future SW.
Whether the films have finished or not, Shrek and the other characters from the films are hugely successful. Universal are still making Shrek attractions at their new parks, and there's no denying there wouldn't be massive interest in such an addition in the UK. If the result of this addition means more people through the gates, ultimately it'll mean more money for original attractions for the future.
Remember this year has seen a huge investment in terms of rides and in particular events, with the likes of the 80's concert, fireworks and improvements to Scarefest. So, to have anything for 2011 is superb news, especially for a Shrek attraction which is likely to need a fairly substantial budget.