Dave, I should elaborate. The 4" vise, at least a Kurt, will bolt up to the second T-slot from the front of an RF table. This puts the rear of the vise just shy from the back edge of the table so you do not lose any travel in Y. There will be times when this matters. A larger vise will project beyond the table in back or extend way out in front.
I own a Kurt D40. It is of a known quality with a lifetime warranty. Glacern is said to also be very good; I suspect it is comparable to a Kurt. I would not go for a cheap vise if I were you.