I'm going to weigh in too because the Gel Res 4 is very dear to me...in fact it's my favorite tennis shoe EVER! Well, them and the Nike Breathe Cage II...
I agree that the Gel Res 5 is a little bulkier and rides higher off the court. It doesn't provide that same locked-in, streamlined fit that the 4 does. However, I've actually switched over to the Solution Speed 2 for tournament play. It isn't quite as durable as the Gel Res, but the ride is similar to the 4 and so is the comfort.
Another one I'd recommend is the Lotto Ultra Raptor IV. It isn't quite as cushioned as the Gel Res 4, but it offers a stable, low to the ground ride with a very natural court feel under foot. It's one of the best shoes around and doesn't get the credit it deserves. However, it does have more of a narrow fit, so if you have wider feet it won't be an ideal selection for you.
In my opinion, nothing stacks up to the Gel Res 4, but those would be a couple good options for you to start with in order to get a similar fit and feel.
Andy, TW