Even with just 1 day in Santorini, you'll be able to see the famous cliffside villages, taste some Greek dishes, and either hike or visit a beach. Spending 2 days in Santorini allows enough time to do a quick tour around the island, sightsee in a couple of villages, and spend an afternoon on the beach or on a boat.
Is 4 days too long in Santorini?
Not at all! Santorini is a bit bigger than most people expect because it's not only Oia. The rest of the island and especially the south, is very worth exploring as well. You can spend a weekend to a full week in Santorini without getting bored, so a 4-day trip is the perfect duration.