In correct Super Rocket spec. (357 cam, high comp pistons, alloy head) Champion recommend N3C, which is equivalent to B8ES or B7ES. I think the NGK numbers go in the opposite direction to Champion, i.e Champion N4 runs hotter (retains more heat / has a longer porcelain insulator cone) than N3 whereas NGK B7 is hotter than B8. I'd start with a B8ES.