I liked the fourth one the best because it had the most variety in color. The greyscale was perfectly fine too, and was better than just the lineart because it showed difference in shading/color which I appreciate. :3 I didn't like the third one as much because the colors felt too flat to me. :/ especially when next to two. xD;;;; Anyway, they're all great, and whatever you feel most comfortable with you should just go for. d^ ^b
Either 1 or 2 depending on the mood you're going for. Flashbacks or hazy cloudy moments (sort of like how Shrike must've felt after waking up from his wounds in the last comic) would use 1 the best but 2 would probably be better for everything else, it suits your bold style well.
Voted for the second one, but I think it really depends on the mood. EXAMPLE while its not good to switch while doing a continuation, use Greyscale for things like flash backs, or for a whole comic if the mood is slightly down (but not dark, like with Black Ace in around the twist worked WONDERFULLY with the color's you used). Two imo gives more life than 3, and greyscale > black and white as well.