I just can't get the 4 kills with Genji's Dragon blade. I tried today but as you saw, I was playing pretty badly with Genji lol. I think the opponent's skill was quite a bit higher than what I usually play against.
Reaper's 4 kills is pretty easy. Maybe play on solo and just jump down from behind. Some times you get stopped instantly, other times you're getting 4-5 kills. Nano boost will obv help a lot.
Yup, Reaper was one of my earliest Ult quad kills. There's almost no warning for it so it's easy to just jump into a chaotic battle and use it.