Help! Baby not sleeping well

Hey guys,so my 4 month old usually goes to bed at 8:30 and needs us to put her paci in for her about 3-4 times if she is fussy until she falls asleep. Well lately it takes her 2 hours to put to bed and she woke up at 4:30 am last night which isn't like her. My husband is still trying to get her to sleep and it's 10:30! She seems tired but she cries when we leave her in her crib. Does anyone know what can cause this and any suggestions on how to help her??? Thanks!