At 4am, presumably entire was asleep. Except X. Except BK.
It would appear that At least two things might have occurred near-simultaneously -- Door Dash arrived and BK was parking.
IF X accepted her delivery in hand or even retrieved it contact-freely, she could have been on the lower level from 4 am to 4:12. She could have sat down on the stairs to eat her food, enoy her tiktok, disturb nobody...
It's possible the K told M "there's somebody here" -- because she heard a sound, heard the sliding door, heard the door dash delivery, heard BK -- and identified the sound as distinct from sounds her roommates might make. (She may have looked out a window, saw a figure, or hearing what she heard, shut Murphy in her room and crossed over to M's room.)
But it's also possible X said it. To Ethan.
Consider: X is on the lower level when BK enters (around 4:06, 4:08, 4:10) and proceeds directly to M's room. X reaches the middle level. 4:12. And something is off. The slider is open. Something. Not necessarily scary at this point, just weird. Weird enough to tell Ethan. Ethan gets out of bed to investigate. Steps into the doorway where he encounters BK, returning from the third floor.
IMO one targeted murder, three reactive murders and a fast retreat.
JMO