Hi, Evan here, thanks so much for getting the game!
Playing with more than 4 players is definitely doable, it just changes the game's balance in a few ways. Before I mention those, you could avoid the issues by having some players play jointly, using a single character sheet, making decisions and roleplaying together. Holmes and Watson, Thelma and Louise, partnered police, an old retired couple - any team that will work together and share stats.
If you instead go with additional individual characters, the game won't actually be too much longer. The main thing to keep in mind is that every player will have fewer turns in the spotlight. That might mean they don't have enough airtime to give their character a satisfying arc. Your group can mitigate that by including more player characters in each scene, and giving them space to play and evolve even when they aren't in the spotlight.
Another effect is that the damage of the case will be divided between more players - so each individual character won't get as bloodied or broken by their investigation. You could just accept that, and play a gentler, easier game. If you'd rather keep things rough, try this: After character setup, for each player beyond the fourth, have every character take one damage to one of their four stats. Choose any stat except the lowest (or tied for lowest).
Thanks again, and please feel free to reach out if any other questions come up! Best of luck on the case! 🕵️♀️