I'd give them a call, I can't see them cancelling the bookings of the travelling passengers but it is usually better to let them know in advance.
That is unless you got a special tariff that is dependant on 4 passengers travelling together, but those are not common any more.
Things might be a little more difficult on the day if only two out of four check-in and you encounter an inexperienced agent (or a third party handler) who doesn't know how to deal with it, and a long queue of passengers.