Why exactly 4 years are the time of a full amount of days?

Exactly each 4 years a new cycle begins.3 years are each 365 days,then the next year is 366 days,which means that each 4 years an exact amount of FULL days passes.Why is this happening after 4 full rounds around the Sun?How is this connected to the fact that the tropic is exactly one quarter of the 90 degrees?