Also, 4-day trips are assigned within all reserves that has 4 or 4+ days of availability, and scheduling will always try to SC you on 1st day, then give you a 4 day trip. If you have 6 days on, and your preferred 4 day starts on your 1st on call day, chances are they will not honor it (unless nobody else in your group can accept that trip).

FWIW, i've been told the way short calls are assigned aren't exactly any black magic or voodoo artform. They actually use the magic "computer" to shoot out a list of possible names, and it's up to the schedulers to figure out who gets which shift of SC.