The 4m comfortably sleeps around 4 adults and the 5m around 6. The 4m is lighter and fits smaller pitches. The 5m gives you noticeably more floor space and a 3m centre height.
At a glance
4m: sleeps about 4 adults, 2.5m high in the centre.
5m: sleeps about 6 adults, 3m high in the centre.
How many people fit really depends on your beds and how much kit you bring. We've had 14 people sitting in a 5m Glawning for a party.