@Gregbyu1 said:

Is there a way to play the base game with 4 humans instead of just 3?

Thanks

The free game is 3 players, you have to earn or buy scrolls to temporarily or permanently unlock more players, scenarios, or expansions.
https://www.catan.com/game/catan-universe

The relevant FAQs (from https://forum.catanuniverse.com/category/13/faq)
https://forum.catanuniverse.com/topic/782/free-auto-custom-match-how-do-they-work
https://forum.catanuniverse.com/topic/4460/how-to-use-scrolls-to-unlock-expansions
https://forum.catanuniverse.com/topic/2635/catan-universe-pricing-and-in-app-purchases