Premier Inn T4 is surprisingly decent and rates generally are cheap as chips. I had a room with 2 double beds (thus could fit 4) for just GBP30 less than a year back!
The transfer really isn't too cumbersome - walk from T5 to Heathrow Connect trains, and on arrival at T4 a straightforward walk through a sheltered tunnel. Of course, nothing compares to the convenience of Sofitel T5 though, but then again what you save on accommodation can be spent on a nice dinner at one of the hotel or airport restaurants.