How to get my 4 year old to go to bed??!!

I have a 2 year old and a 4 year old that share a room. My 2 year old goes to bed first at 7:30 and then my 4 year old used to go up at 8pm (they cant go together because its a nightmare!)

Anyway my 4 year old now stays downstairs with us until she falls asleep on the sofa and then we take her up, this is at abouy 9pm. She refuses to go up to her to bed if we try at 8pm when shes awake. She will scream and cry and obviously would wake her sister up so we dont know what to do?? No amount of bribery works! If she was in her own room i would just make her and let her cry but i cant! Help!!