You should be able to find plenty of activities that your 4 year old can do. Ziplining and some of the other adventure tours like rafting, canyoning, etc. are better for older kids but there’s lots of other things to do in Manuel Antonio and La Fortuna for kids. Your family would probably enjoy a night tour in La Fortuna and the hanging bridges and many of the nature tours offered in both destinations. We actually have a family-specific itinerary in our Top 10 Costa Rica Itineraries book that includes lots of ideas for things to do with kids in Manuel Antonio and La Fortuna so you might want to check that out. You can find the book on Amazon here. Hope that helps!
Answer for Zip Lining/Activities with Kids