By the way: sorry to pester you, but I've got doubts, if a difference between 2x57mm (HE kill 8) and 4x57mm (HE kill 10 - only 25% better) isn't too tiny, for twice as much rockets... On the other hand, 8x57mm has kill 15.
As for now, two shots of 2x57mm are much deadlier, than a single shot of 4x57mm - and some units had a plethora of 2x57mm (like Russian #965 Mi-8TB with 96 shots - 192 rockets), what gives them some sort of advantage.
I suggest to give:
2x57mm HE kill 6 (or 8),
4x57mm HE kill 12,
8x57mm - some 18 (or more), considering a big number of rockets in a salvo.
Two rockets would be pretty lousy choice then, but 57mm S-5 weren't regarded as highly precise and efficient weapon anyway (and there would be still a worse weapon: French 2x37mm SNEB with kill 5). 4x57mm could be considered a primary way of using these rockets then.
BTW: 2x57mm rockets are used eg. by Soviet #364 Mi-24A, which has 64 rockets as for now, while it carried a standard Mi-24-family load of 128 rockets (4x32, photo
http://imgproc.airliners.net/photos/...5680.jpg?v=v40 )
For comparison:
2x RP-2 Rocket has kill 8, 4x RP-2 Rocket has kill 12 (50% better)
2x 68mm SNEB has kill 11, 4x 68mm SNEB has kill 14 (27% better - still not much)
2x 2.75in FFAR has kill 15, 4x 2.75in FFAR has kill 20 (33% better)
2x 70mm Hydra has kill 15, 4x 70mm Hydra has kill 20 (33% better)
With HE kill 12, 4x57mm would be similar to 4x RP-2 (2in), only marginally better than 2x68mm SNEB and worse, than 2x70mm, what might be close to reality.