This seems like a Criminal Minds episode. If the attack happened at the time they are reporting it is possible the killer killed all of them while they slept. If they had any knowledge of anatomy and was strong enough it could have been done with very minimal sound.
Something I noticed is all 4 were in a fraternity or sorority. It could have been someone that targeted the group because of that. Maybe they were hazed at some party in the past or not allowed in. It is also possible the other 2 were sleeping in a different part of the house or the killer wanted to cut the chance of being caught. If the other 2 were larger maybe they knew they couldn't overpower them.
Lots of questions at the moment.