It's great if you're into surfing. The streets aren't paved, so it gets dusty. Watch the sunset on the beach. You might even see wild horses.
The location is more remote, so if you have 4 weeks, you could probably explore more of Costa Rica. Then again, it takes some work to get there. You probably need to catch the ferry, or rent a car that can handle rough roads and potholes. Depending on where you're starting, there might be a bus you can take.
This trip really depends on your personality. Santa Teresa has a chill vibe, so if that suits you, you could stay the whole time there and just relax, lie on the beach, surf, or do yoga.
Here's a good article on Santa Teresa
with a list of restaurants, hotels, and other details.