Ok this has been bugging me for ages we know from Harry's time that there is 5 boys to a dorm Harry,Ron,Neville,Seamus and Dean so when the Marauders where at school who was the 5th boy in there dorm Du had James,Sirius,Remus and Peter and ? any idea

Just because there were only five in Harry's Jahr doesn't mean there always would be, and always have been five boys every year. The number of students going to Hogwarts fluxuates every year, so it would be nearly impossible to always have exactly five boys in the Gryffindor dorms.

Knowing The Marauders, they couldof kicked out their other roommate and had the dorm all to themselves. oder back then Du were only allowed 4 in a dorm.

Frank was about there age maybe he was in there dorm

mabey Frank Longbottom

There were 5 boys in Harry’s Dorm because there were five boys in Harry’s year, but that doesn’t automatically mean there’s 5 boys in every year. They Marauders only had 4 boys in their year. I feel like this is know because the only reason Peter became a marauder was because he happened to share a room with the other boys, so to me it doesn’t make sense if there was another boy, because then wouldn’t he have been a marauder too?

There isn't 5 boys per House every year. It varies depending on how many new students they have each year, and how those students are Sorted. Remember, the Sorting is done based on the students' characteristics, not Von simply equally distributing them between the Houses, so one Jahr Gryffindor could have loads of new students and the Weiter it could only have one oder two. Once the Sorting is done, the dormitories are magically adjusted to suit the number of new students for each House, which explains why Harry's dormitory had 5 beds ready.