While Young Sheldon season 4 ended on a cliffhanger regarding George Sr.'s potential affair with Brenda, The Big Bang Theory already answered this lingering question. It's been two years since the long-running sitcom TBBT wrapped up its run with 12 seasons. Since then, its spin-off has become CBS' premiere comedy. While it can mostly stand on its own, Young Sheldon season 4 was more active in strengthening its narrative connection to its parent series.
Previously, Young Sheldon has been lax about honoring established continuity in the franchise. It's one of the reasons why not all fans of The Big Bang Theory enjoy the prequel. But, that's seemingly changing as the series moves forward, with season 4 featuring various references to its predecessor. For starters, Amy Farrah-Fowler (Mayim Bialik) made a voice cameo during the premiere, where it was revealed that she and Sheldon share a son named Leonard Cooper in the far future. By the end of the year, Young Sheldon also laid the groundwork for George Sr.'s (Lance Barber) infamous cheating storyline. The traumatic narrative was first revealed in The Big Bang Theory season 10 while the socially inept genius explained his obsessive need to knock three times.
For years, Young Sheldon tried to dance around this particular plot — understandably so, since it's a bit dark for a family-friendly sitcom. But as the show runs out of time with regard to the timeline established in The Big Bang Theory, CBS will have no choice but to find a way to incorporate it in its future storytelling. In the season 4 finale titled "The Wild and Woolly World of Nonlinear Dynamics," Mary (Zoe Perry) and her husband get into a heated argument, resulting in George admitting that he's not happy with the way his life turned out. Instead of revealing more of his well-guarded feelings, the Cooper patriarch goes to a bar in an effort to let off some steam. That's when their newly single neighbor, Brenda Sparks (Melissa Peterman), showed up and offered to keep him company. The screen then goes to black, effectively hinting that this might be the start of George's infidelity storyline. However, The Big Bang Theory has pretty much confirmed that Brenda isn't going to be George's mistress — otherwise, Sheldon would've name-dropped her while she recalled the horrific story to Penny (Kaley Cuoco).
As someone with an eidetic memory, not to mention keen attention to detail, Sheldon would've mentioned that he caught his dad having sexual relations with their neighbor. Before his confession to one of his closest friends, he had mentioned Brenda's son, Billy Sparks (Wyatt McClure) repeatedly over the years, meaning if he remembered him, chances are that he also remembered his mom (more so if he caught her in bed with his Dad!). The fact that Sheldon didn't mention the name of the woman his father cheated with implied that it wasn't someone he knew — or at least someone he is willing to talk about. Had Brenda been the one to break apart his family (in his eyes), it's unlikely that Sheldon wouldn't be so cavalier about referencing her son.
In terms of the timeline, it also makes sense that Young Sheldon isn't rushing into tackling this narrative. The sitcom has at least a couple of years before the incident happened, so there's no need to fully focus on it in season 5. For what it's worth, Young Sheldon may not even immediately answer the lingering question that the season 4 finale left. Instead, it could execute a minor time jump and shift interest to a different, more palatable, topic. The prequel could even defer tackling the cliffhanger for an extended period of time so it doesn't bog down the show's humor and lightheartedness. That being said, unless it wants to contradict what's been established in The Big Bang Theory, the spin-off will have to eventually show George Sr.'s affair.