It's more like the movie Elysium. There was a group of mice that separated themselves and just maxed out all day, but they forgot how to reproduce and care for their kids, so once the population hit it's peak and started declining, that group were the last to die off, but they couldn't bring the population back up.
4 males and 4 females isn't much when it comes to genetic diversity. Depending on how many generations the colony lived, inbreeding might have been the cause of extinction.
https://www.smithsonianmag.com/smart-news/how-mouse-utopias-1960s-led-grim-predictions-humans-180954423/