What is average menstrual period length? is it normal to vary 2-4 days either way each month?

Yes: Varying menstrual bleeding length is very common. The physiology textbooks say that a normal cycle is 28 days with 7 days of bleeding. In my practice, 3-5 days is the usual answer for period length.

4 to 5 days and yes: Normal length can be 2 to 7 days, and can be even shorter if someone is on birth control pills. And yes, cycle length can vary month to month.