On high bar, the main focus should be on tap swings. Otherwise, it's more or less the same as girls. There's really only three skills in the level 4 boys routine: a pullover, a cast undershoot (and they're less picky about cast shapes on the men's side; they don't have to be hollow; they just have to have tight legs), and tap swings. I believe more than half their score comes from their tap swings.
Pommel horse, the main focus should be on mushroom, and there really aren't many drills for that; you just gotta have them do a million circles, reminding them to keep a tight and extended body. Ideal position for a circle is as extended as possible; the natural tendancy is to hollow, so I tell all my boys to try to do their circles in a tight arch, with the hips lifting up in front and the heels lifting up in back to try to combat that tendancy. As for the horse itself, anything you can have them do that involves leaning and shifting their weight while keeping arms straight will help. THe level four routine itself is basically just a couple of drills strung together.
Rings is a bit tougher. The strength elements at level 4, while not necessarily easy, are not particularly technical, and are pretty self-explanatory. The swings are the hard part (and since they just changed the definition of an ideal basic swing, we're all sort of scrambling to figure out how best to teach the new swings). The idea is that in the front swing, the toes should come up and over the top, almost to inverted hang, but pressure should be applied outward and backward near the top to stall the swing and stop it from reaching a full inverted hang. In the back swing, the heels should rise as high as possible, and the chest should remain pressed down to whatever extent possible; position should be like a reverse candlestick, with the hips and lower back more or less straight, and most of the bend should be in the chest.
Another thing to watch for: as the gymnast swings backward, the hands should turn so the palms are facing out, and once the swing gets powerful enough, the gymnast should push the rings out to the side a bit (though the chest should remain the lowest part of the body, and shouldn't rise more than necessary. As the gymnast swings forward, the hands should turn so that the palms are facing in and slightly back.
Hope this helps!