The obvious answer to your question, is that one guest would not be able to receive the service and you'd fail their task.
4 customers do not fit in a room which caters to 3.
But you'd need to be extremely unfortunate, or extremely stupid, to take on a 4th guest who required the same service, at the same time, when there are only 3 slots available in the room.
Even then, you can just refuse the guest, at reception. Or you can reload the autosave from the start of the same time period, get the same guest turn up & they'll require different services.