In the past when I've written out of order, it's never really stuck or worked for me, and it's not something I particularly like doing. I have a strong suspicion that the only reason that it's working out here is because the four plot threads are so separate that I don't have to worry terribly much about one affecting the others, at least at this point -- normally in the past where I've written out-of-order, once I've written the candy-bar scenes, I don't want to go back and write the build-up! (I don't remember who called them candy-bar scenes -- the scenes that you're really looking forwards to writing, the reward type scenes, rather than the build-up. For me it's normally fight scene, heh.)