You can also cancel the 3rd swing foward at any time in its swing to get the 4th swing out faster, its a little awkward to get used to because it changes your timing, but super useful if you get it down. You can also use the first swing in the air to **** with spacing in certain match ups becuase of the stall. Its probably more useful with Marth, but im sure it can still be used to mess with aproaches. I also think that the first swing is his strongest (if not only?) option for jab resets. Also yeah, XX> is probably the worst of the possible combinations (though its by no means bad). Since you can cancel the third foward, I pretty much always use XXV or xx>(canceled)> when im trying to kill. 3down has a weaker knockback, but the angle is so much stronger for fast fallers, and since (in my experience) floaties can smash DI out of DED pretty easy, its the only time you should be using it to kill.
as for the 4th swings, up is really only useful when they DI so that the third swing puts them like *on* roy or above him, since the hitbox is the closest to him, 4th foward is a kill move, i usually pound it out of a third foward cancel since the hit box covers more, and the 4th down is super useful for sheild damage, since it has lots oh hits and its hitbox points down, so it can poke really well.
generally speaking, just mix it up constantly. Roy has so many good optios with ded and its important to not rely on only one of them, as they're all fairly punishable given your opponent knows how to react to it.