1)
glitterbox is classic house
insane is tech house mostly
el row is tech house to techno, depending on acts
pure pacha is house and electro - the most 'commercial' night out of all four
2)
glitterbox is terraza only - this is to be noted, so you won't see the full of space. terraza is busy though
insane is normally perfectly busy, not packed
el row is perfectly busy - discoteca and terraza, plus sunset terrace is also nicely busy mostly
pure pacha is normally easily the busiest of those four nights
3)
I'd say there aren't any big differences really.
4)
depends heavily on your tastes. el row is a very fun one though.