4th is about right, if you've not flown for a while you'll also find the second charge is better than the first, with my Wattage 1100 hydride pack it likes to be flown still warm from the charger, ni-cads also give a bit more in this warm state.
one thing that may make a differnece, packs give in the same manner as they were charged. if you slow charge a pack you'll get a long run time, but at a lower power level, if you fast charge, you get more punch, but less duration. i know it shouldn't be, its a battery, it has charge, how can that be? but i know first hand from racing its true, if i had to quickly race and wacked the charger upto 6.5A i'd get a hell of a kick for the first few laps. it might just be because of the extra heat generated.
what can you tell us about your battery? and whats a Airstrike? apart from a accident