Hi Karen,
You’ll get in some good beach time in Cahuita so maybe head to the mountains (Arenal or Monteverde) for the other part of your trip since it will have a very different feel. Between the two, both are great destinations, but with 4 days, we’d say Arenal is probably your best bet because it has more to do, especially based on your list (it has rafting, hanging bridges, good hiking, etc.). Here’s a blog post we did with more info about what the area is like, hotels, etc.: La Fortuna: What to Expect from Costa Rica’s Most Popular Destination. We also have a post about Monteverde if you’re still not sure: Monteverde: A Forest in the Clouds.
Hope that helps!