You could either use a nested conditional branch that would look something like this :
Conditional Branch : Ralph is in the party
-Conditional Branch : Ulrika is in the party
--Conditional Branch : Bennett is in the party
---Conditional Branch : Ylva is in the party
----Control Switches : [0001] = On
Or use a script like this :
AcN = $game_actors[1]
A = $game_party.members.include?(AcN)
AcN = $game_actors[2]
B = $game_party.members.include?(AcN)
AcN = $game_actors[3]
C = $game_party.members.include?(AcN)
AcN = $game_actors[4]
D = $game_party.members.include?(AcN)
if A && B && C && D then
$game_switches[1] = true
end
Where the numbers displayed within '$game_actors[1], $game_actors[2], etc' are the index numbers of the actors you want to check for. As in, the number displayed on the Database window under the Actors tab. The script could probably be made better, but that's the basic idea and it should work.
EDIT : Oh, you're only wanting to check for a certain number of members? Oop, I guess your question came across wrong to me. You can try this for determining if you have four people in the party currently :
if $game_party.members.size == 4
(Do Stuff)
end