Assalamu Alaikum
Listen brother, the Bible tells us that prophet Solomon had 1000 wives, actually 700 wives and 300 slave women!
As for your question, why he was allowed to have more then four, and others only 4, it is simple, the ruling was not yet there, Allah has ruled the people diffretnly throughout time, diffrent Shariah and so on, some say Alcohol was allowed, some say it was allowed to prostrate yourself for your father or mother and so on, all of this is big sins in islam!
Why diffrent laws? This is only known to Allah, but we can speculate day in and out, why oh why, it doesn't even matter, the point to note here is that things change and people change too, diffrent laws where maybe needed then, but this Islamic law is to be followed until doomsday, no changing!
People may Ask why did Muhammad(saw) have more then 4 wives, this answer you can find in
www.muslim-responses.com or Answering-Christianity.com akhi, May Allah bless you ameen