I own both.
I enjoy plinking with the 5.5" more than the 4". I can run a hundred rounds of mid-grade magnums through the larger one with less fatigue.
I'm also a bit more accurate with the larger one.
Differences:
I've got a Hogue hardwood monogrip on the 5.5", as well as a V-notch rear sight and a fiber optic front sight.
The 4" has stock U-notch rear sights with orange ramp front, and stock rubber grips.
I carry the 4" for hiking in high country or when out hunting.
Truth be told, the 4" was an impulse purchase when they first came out. It's a great sixgun and I enjoy it, but my needs were met by the 5.5" gun already. Nothing wrong with more than 1 redhawk, though.
If you're getting only 1, I'd suggest the 5.5" one.