With just one night, we would recommend either adding a night in Tamarindo and exploring some of the beaches up and down the coast on day trips (Playa Avellanas, Playa Conchal, etc.- more info in the links) if you want more beach time, or stopping in the Rincon de la Vieja area on your way to Monteverde. There aren’t a ton of places worth visiting between Tamarindo and Monteverde, but as we were just telling someone else in this forum, Rincon is worth a stop if you’re interested in seeing an authentic Costa Rican town/area and visiting a national park with volcanic features. Check out our response to this forum question for more info about Rincon.
We wouldn’t recommend adding a 4th night in Monteverde whatever you do- Monteverde is awesome but 3 nights is really enough time there given the activities available. Hope that helps!