crete

I would split your time, stay to the North coast, depending on where you might be coming from or going to. The two major cities would be Heraklion and Chania. Chania, to the west is a great place to kick back, great old town, good restaurants. It can be used as a base for the Samara Gorge, but that will take an entire day. I could do 2-3 nights here easily, including some side trips to beaches. Heraklion is not a very scenic city, but is worth it for Knossos and the Archelogical Museum, as well as the main arrival and departure point for the island. Chania also has both Airport and ferry service. If you squeeze it in, Rethymno in between the two cities is rated pretty high on some peoples list. All three are on a single cheap well serviced bus route.

Crete is huge, over 125 miles across. You won't be able to explore very much in such a short time. I suggest you choose one destination, rent a car and take daytrips to nearby points of interest. Western Crete is beautiful. Stay in Chania. But if you want to see Knossos stay in Heraklion, then you can drive to the south shore to Matala and Agia Galini.

The major city is Heraklion. It's not much to do there besides bars a harley Davison place and a few tatoos places. Renting a car and drive around the island is the best way to see it. The views are stunning so the smell of the olive trees.
We stayed in Heraklion for a night and did a tour of the island in a small Fiat. We stopped in a few times and had coffee and salads with a lot of Feta cheese.

Did you mean 3 nights/4 days?
BTW I wouldn't spend more than 2 days there.

Bea: Did you mean you wouldn't recommend spending more than 2 days in Crete overall or more than 2 days in Heraklion? We're planning a trip to Greece and are considering Crete as well. Thanks!