To unlock the No Traitors trophy in Red Dead Redemption 2, you need to finish Chapter 5. There's nothing missable in terms of trophies in Chapter 5 so don't worry too much about mopping everything up. In particular...
*** Spoiler - click to reveal ***The flora and fauna on Guarma don't count towards Compendium completion.
It's worth noting that the Chapter doesn't come to a complete end until well after you return from Guarma until you have settled into your new camp in the north, so you may end up unlocking some new free roam activities and Stranger missions in Chapter 5 that are normally labelled as Chapter 6 exclusives.