Why is putting a 10 year old boy and a 4 year old girl in the same room even an option? Why is there not an option to bunk the baby with the 4 year old? They are both girls and closest in age.
I was one of five growing up, the second oldest and middle girl. I NEVER got my own room and I can tell you it's something we still talk about today. My parents made the right choice though. My older sister is 2 years older, then two boys that are 2 and 4 years younger than I am, and then a sister 6 years younger. It made the most sense to bunk the two older girls, the two boys, and the baby girl in the three rooms.
My oldest and middle (girl and boy) shared a room for a year when they were 3 and 1 because they wanted to. They each had their own rooms, but they always bunked together in that house. It was okay at that age - not now and they are 10 and 8.
Your best bet is to give the 13 year old a room (the second biggest of the 3 non-master rooms), the 10 year old the smallest room, and the 4 year old and baby the biggest of the three rooms. That is how I would do it and I wouldn't even think twice about it.