I would agree that with 4 nights (which probably translates to 3.5 days) there's plenty to do in the 5 Lands to fill your time, and especially so if you intend to do some hiking. You won't be able to hike between ALL of the villages on the best-known trail (#2 Sentiero Azzurro or "Blue Trail") as parts are currently closed, and at least one of the segments will remain so this year. You may, however, do whichever of those are open and jump the train to the next village.
That aside, there are other, upper trails to hike, and which offer wonderful views. Some of the cemeteries and churches are climbs to the higher reaches of the villages for terrific views as well. And if you're going to be there during the season the ferries are running, getting a look at the coastline from the water offers another nice perspective.
We had 3 nights and wished we'd had more. Our time sped by all too quickly!