I mained ROB in brawl and now I'm maining him in Smash 4.
In brawl one of the best ways I found to get kills was getting someone off the edge and using my forward aerial over and over, with second jump and sometimes with the upB as well. It wasn't a true combo but it was faster than most other characters could respond to (a frame trap).
There are a few factors that I suspect may be the cause but I'm not sure. The hitbox looks to be much higher than in brawl, and perhaps the angle it launches you at is harder to follow up on. But I would have thought that the much higher hitstun would mean this is much more forgiving than in brawl.