You only need to land hit 3. You can just learn to deliberately space the move as such so that you land immediately after that hit connects.
Hit 1 only also works, but only at KO percents (although that is exactly why you want the combo anyway lol, but the setup into D-smash will stop working at percents where you might consider killing with U-tilt), and it's much harder to space into sweetspot D-smash range. If you land hit 2 and don't connect into hit 3, you can't actually combo into anything btw.
Hit 4 can also combo into D-smash, but at 0% basically lol.
Hit 3 is awesome for having set knockback.