Tarmach,
There is typically a 2 year degree called associate level and then the 4 year degree which is a bachelor degree. The airlines don't want the 2 year degree so they say a 4 year degree is required. The education structure is a little different over in the States and there really is no equivalent to a 3year degree similar to the UK.
The requirement is just another way to be selective, several airlines have already dropped the hard requirement but say it is desirable. I believe around 90%+ of applicants hired by the Majors in the states do have a 4 year degree.
Regards
FYF