This may be the noobiest question I've ever asked, but how does one adjust a hop-up for optimal performance? My BB's from my M4 seem to drop. What way do I adjust the hop-up, and how? What sort of tool do I use? Do you adjust it by turning the phillips head screw that is inside of the gear under the dust cover?
Nope, you turn the cog behind that screw. Turning it clockwise increases hop, and counter clockwise decreases hop. Quarter turns are good to start your rough adjustment, but once you're in the ballpark of "optimal," just nudge it back and forth. As for what actually is optimal trajectory, that comes down to personal preference a bit. Generally, as flat for as long as possible is what you're looking for, because your shot will line up with your sights for a good long time. Other people like to actually arc their bbs up just a bit because you get a bit more range out of the gun. If you do that though, it's a little harder to shoot accurately because your bb's path is a little more of an arc. Which style you go with is up to you. And of course, it's a sliding adjustment, so you can always mix the two approaches.