Welcome to FT.
Have a look here, but remember that surface segments must now be counted in the 20.
http://www.flyertalk.com/forum/showthread.php?t=411736
Which continent would be the 4th? Big impact on maximum routing. Time of year? Big impact on flights like DFW-ANC etc.
AA Q-miles/points? BA Tier Points? QF Sector Credits? More big differences.