As they're left together at nursery, some people will assume their 4 yr old is OK to be left with their nursery friends for a few hours. But I do think it's on the young side, and I would never ever have left mine at that age in a large hall-type party - I'd have considered it for drop-off at someone's house party (usually smaller numbers).
Parties my DC have had:
4th: hall party, one person wanted to drop & run (coincidentally the non-RSVP-er...)
4th: house party, everyone stayed (mates of mine mostly anyway!)
5th: party at home, informed parents they'd be dropping (small house!)
5th: hall party, everyone stayed - 1 person checked it was OK to leave for 30 mins before her DP could get there - totally fine with supervising 1 extra child, but not 40!
6th: small tea party, regular playdate friends only, drop off
6th: hall party, everyone stayed
By 7th, most people are going to smaller groups anyway when you wouldn't expect them to hang around.
Usually as PPs have said by school-age you need to consider siblings, and then it can be better to have people drop off rather than stay for that reason, otherwise you end up with more than you bargained for.