After comparing the two (SR4 HH and SR5HH), I'd bet the farm that the SR5 HH is wired series on the humbucker coils and the SR4 HH is parallel. To me, it sounded quite obvious (at least in the bridge 'bucker mode). I'd also wager that the HS's are the same on the humbucker wiring.
Stingray 4 = Parallel
Stingray 5 & Sterling = Series