I'm no expert, but this is what my gut says:
Someone unknown to any of the victims (or the other 2 housemates who were not physically harmed) was stalking at least one of them, unbeknownst to any/all of the victims.
I think this person has killed before, likely with the same weapon that was used. This is not the kind of weapon an inexperienced or opportunist killer chooses at random - and is successful with! - to commit their first (4) murder (2). They successfully killed 4 young adults (i.e. prime physical condition) with a knife. IMO, they knew exactly what they were doing, how it would feel to do it, how much pressure to use, how much time it would take, etc.
Also possible they've used that knife to cut food (or other bodies - sorry! - after prior murders). I think this knife is the perpetrator's companion!
Either this is their weapon of choice (my main theory) OR the perp is barred from purchasing a firearm and that's why they chose the knife. If the latter, this, to me, indicates the perpetrator is likely a loner - because if they had other nefarious friends who might have guns, someone could give/loan them a gun or they'd steal/borrow it.)
Knives are more difficult to trace than guns because there are no bullet casings left behind. The killer knew this.
When LE says it was a targeted crime, I think that means they believe the perpetrator chose a person to attack, watched them, stalked them (probably unnoticed), planned the attack including how to get away.
I think they are indicating that there's not much of a risk the perpetrator will kill someone randomly - but that doesn't mean they don't have other PLANNED targets in town.
Having stalked them, I think the perpetrator knew there would be multiple people in the home and was prepared to kill any/all of them. But I think they killed ONLY those that they'd witnessed coming back to the house during those early morning hours. I imagine the killer lurked nearby in the wooded area starting sometime after dark, watched as the couple and the two friends returned home from hanging out and attacked them because they saw them/knew they were in the house.
Personally, I'm convinced that the other 2 housemates were in the house/asleep from much earlier in the night and weren't killed simply because the killer did not see them THAT night. I think the killer knew for a fact that those 4 were home because they were watching the house but didn't bother to search for additional housemates.
As creepy as this is, I think it's also possible they did this to scare the other housemates, as in they killed these 4 to send a message to one of the other 2 (who is their REAL target - psychologically).