Marital union is a big one. You must be living with US citizen spouse in marital union for over 3 years at time of filing N-400. Yes, you were married for 4 years, but did you live separately at any point in the last 3 years due to you being overseas and spouse being in the US? Some officers interpret marital union very strictly, denying such cases if filed within 90 day early filing window. If you file at 3 year anniversary of living in the US, that usually gives no chance to deny your case on this requirement.