I cook loin backs, St. louis trimmed spares and untrimmed spares. They are all good :)
As mentioned, loin backs will take less time. I look for racks that are 2.5 - 3 lbs. For me they take 4 1/2 hours or so at 260 - 270.
Are you really getting untrimmed spares done in 4 hours at 250? They must be small racks. The smallest untrimmed spares I can find around here are at least 4.5 lbs.