4.5" x series is still short arms. short arms are control arms that still mount to the stock locations. once you go above 4.5" of lift, i'd say you need long arms. i wouldn't hesitate to get a 4.5" lift with short though.
aftqer 4.5" of lift, your control arms look nearly vertical, with no room to swing down for flex. long arms are control arms that mount to a cross member that is mounted farther underneath (usually where the tcase crossmember goes) so they have more room to travel.