Broly has been a staple of the Super Dragon Ball Heroes manga since its inception, receiving several new transformations and power-ups, and his Super Saiyan 4 form has revealed the pinnacle of his power—threatening time itself. Though he comes from a different timeline, the Broly of Super Dragon Ball Heroes evokes the Broly from Dragon Ball Z's early movies, where he was meant to be the strongest adversary the Z Warriors ever faced. When Broly returns in Super Dragon Ball Heroes Dark Demon Realm Mission, he manages to hone his iconic blood curdling power into the ultimate Saiyan transformation, Super Saiyan 4.
Super Dragon Ball Heroes Dark Demon Realm Mission introduces readers to a version of Broly who has been corrupted by a “Dark Mask.” These masks bend the user’s mind, leaving them available to be brainwashed and controlled by the Demon King Mechikabura. They also greatly amplify the wearer's power beyond the body’s normal constraints, seemingly in proportion to the amount of malice within one’s heart. Broly’s power-up after being controlled by the mask is inconceivably large, as the darkness within him seems endless. Broly has always been a tortured character and the Super Saiyan transformation only serves to exacerbate the malevolent desires present within him. Adding the Dark Mask on top of this already bloodthirsty character creates a quick recipe for disaster and put the entire Dragon Ball timeline at risk.
In chapter 9, Broly remains a Super Saiyan 4 while wearing the mask and even though he should be under Mechikabura’s control, his rage causes him to follow Goku into a different space-time. Upon being trapped away from Mechikabura, Broly descends into madness and begins to battle Goku, who immediately transforms into his Super Saiyan 4 form. Interestingly, Goku notes that the Supreme Kai of Time warned him about using Super Saiyan 4, telling him that his power within that form may damage the fabric of time and potentially warp history itself. Later on while spectating, Mechikabura comments on the fight, stating that Broly and Goku’s power is almost the same. Though being able to transcend the power of time is par for the course with Goku at this point, this is the first time Broly has ever been shown to also be capable of potentially causing damage to history.
It’s currently unknown whether Broly can only pose this threat to time because of the Dark Mask, Super Saiyan 4, or a combination of the two. Dragon Ball characters have repeatedly been shown to be able to affect abstract concepts such as souls or time with enough power. In Super Dragon Ball Heroes specifically, characters on the level of Xeno Goku or Demon Gods like Mechikabura have been shown to disturb the flow of time just by existing. Goku's mention of the Supreme Kai of Time's warning pretty much confirms that his level of power, at least upon transforming into Super Saiyan 4, is enough to begin affecting timelines. Broly, now in his Dark Super Saiyan 4 state, should also possesses this level of power that inherently affects time just by existing. Beyond that though, Broly's ki has always been portrayed as a never-ending well with no discernible limit that constantly increases as he fights. His ever-rising power coupled with the Dark Mask and Super Saiyan 4 makes it extremely likely that Broly's state of being alone transcends the limitations of time and poses a threat to the fabric of history itself.
Broly’s Super Saiyan 4 transformation is potentially one of the strongest Super Saiyans in all of Dragon Ball. His mere presence holds such a dark and staggering aura that it's almost frightening to think of how the Dark Mask is amplifying the evil in his heart. That combined with his vast ki that constantly increases at all times means this version of Broly could soon pose a threat far beyond the level of even Dragon Ball's gods. With the recent premiere of Super Dragon Ball Heroes: Ultra God Mission, Broly’s Super Saiyan 4 form will surely be making a reappearance soon enough.