I'm sure one of my Gatwick ex-colleagues will respond, but a few basics which apply anywhere..
Holding patterns are not (usually) designated for particular types of aircraft so the patterns used by Gatwick will be used for anything from a light biz jet to the biggest airliner.
Separation on final approach depends on many factors, weather, day or night, other traffic, etc. The exact spacing is determined by ATC and achieved by instructing pilots to fly at particular speeds which they are required to maintain as accurately as possible. 4nm maybe a basic separation, but this will be extended, possibly up to 10nm, for wake turbulence reasons or to accommodate departures from the same runway. Aircraft are divided into weight categories for wake turbulence purposes and those categories determine the spacing to be achieved by ATC.
HTH