Borderlands 4 may aim to make up for BL3's villains' shortcomings with a darker and more dangerous antagonist, in the vein of Jack. Gearbox clarified that Handsome Jack won't be the main villain ...

Handsome Jack ridiculed you, hunted you, and killed your friends in Borderlands 2. But in the Pre-Sequel, you're going to be best buds. The upcoming game is all about Jack's descent into villainy.

Update: Well, about twenty minutes after publishing this post, Gearbox’s Randy Pitchford nuked this entire theory, saying that the character in question in the teaser is not Handsome Jack Well ...