Warning! The following contains spoilers for the Young Sheldon Season 4 finale "The Wild And Woolly World Of Nonlinear Dynamics." Read at your own risk!
Young Sheldon's Season 4 finale had a downer ending, and viewers may not be aware just how dark things could get in the CBS series going forward. The finale showed a big fight between George Sr. and Mary Cooper and set the stage for an uncomfortable story first revealed by Sheldon Cooper in The Big Bang Theory.
After the fight, George Sr. was next seen at the bar, setting up a game of pool when he was approached by the newly single Brenda Sparks. Both briefly talked about how they just needed some time out of the house, and Brenda asked George if he'd like some company. George agreed, and while it seemed rather casual in the episode, the context of the situation surely set off alarm bells in the minds of The Big Bang Theory fans.
In Season 10 of The Big Bang Theory, Sheldon confessed to Penny the reason why he always knocked multiple times before entering a room. Sheldon said that when he was 13, he walked in on his father with another woman. Sheldon is only 11 in Season 4, so it's possible fans just witnessed the beginning of George Sr.'s illicit affair that Sheldon eventually discovered. At the very least, we saw that George and Mary have clear problems in their marriage, and George doesn't mind the company of other women when his wife isn't around.
This Young Sheldon reveal came a week after another reveal in which Adult Sheldon (the narrator) acknowledged his father's unexpected death was on the horizon. Obviously, we know that George Sr. will still be alive when Sheldon is 13, so his death is still down the road. The period of affairs and infidelity, however, seems to have begun, which may make George Sr. a much less likable character in the eyes of the audience.
Young Sheldon fans shouldn't expect a massive tonal change in the series, though the series has an obligation to tell the stories (sometimes with mistakes) Sheldon already told in The Big Bang Theory. This means we can expect more bitterness between Mary and George Sr., affairs, and yes, the eventual death of Sheldon's father. Definitely a bummer, but I'm sure the series will find some ways to lighten all the dark times ahead for the Coopers.