The Marvel Cinematic Universe has introduced a lot of villains over the last nine years, most of whom have been killed off. Brock Rumlow is one of those baddies. Debuting in Captain America: The Winter Soldier and taking on the Crossbones moniker in Captain America: Civil War, he met an explosive end in the latter movie, but as those who read comic books will tell you, death isn't always permanent in these kinds of stories. With regard to Crossbones, apparently we may see him back up on his feet during Avengers 4.
Take this with a massive grain of salt, but according to a Redditor claiming to be an extra on Avengers 4 who has been posting behind-the-scenes pictures from the set, they saw "the guy who plays Crossbones," a.k.a. Frank Grillo, on set. Assuming this is true, it's unclear how Crossbones will play into Avengers 4. If he weren't dead, I might suggest he's part of a side narrative, but bringing a bad guy like that back can't simply be brushed off in such a way. Of course, then there's the question of if he's even been resurrected of if this is part of a flashback.
In Captain America: The Winter Soldier, Brock Rumlow was the leader of STRIKE, the S.H.I.E.L.D. counter-terrorism unit whose members were later revealed to be HYDRA agents. During Winter Soldier's climactic conflict, a Helicarrier crashed into the building where Rumlow was fighting Falcon, and Rumlow was badly wounded. Returning in Captain America: Civil War, Rumlow, now covered in scars and going by Crossbones, had ditched HYDRA and was carrying out his own terrorist attacks. The Avengers intervened in his Lagos operation, and Crossbones died while attempting to kill Captain America with a bomb, only to have Scarlet Witch contain the explosion.
You'd think that would shut the book on Crossbones' time in the MCU, but last month, Frank Grillo implied that he might not be done playing Crossbones after all. The actor later clarified that there are no plans for him to return to the franchise, but assuming this Avengers 4 set information is legitimate, then Marvel may have found a way to bring him back in. However Crossbones fits into the picture, if at all, it certainly isn't good news for the heroes.
Just like Avengers: Infinity War, plot details for Avengers 4 are being kept under wraps, though because this is the last movie of Phase Three, it's expected to be equal in scale to Infinity War, if not bigger. Along with the Crossbones information, this same post also said there was a scene being filmed for Avengers 4 where Tony Stark is in a B.A.R.F. (Binarily Augmented Retro Framing) simulation (just like the one in Captain America: Civil War) with Harley Keener, the kid who helped him out in Iron Man 3. The two of them are studying the Battle of New York from The Avengers, with Tony trying to figure out how to use the Tesseract against Thanos. Let's hope Crossbones doesn't interfere with this strategy session.