That's a long way to drive for just 4 days of the Keys. There are so many different things to do and so much more than just Key West. That is one of my least favorite places in the Keys.
Marathon has lots to offer, from turtle hospital to dolphin research center to the Fl Keys Aquarium. There are a couple of really nice state parks in the Keys that you should check out. Key Largo, Islamorada and Big Pine Key have lots to offer.
Get on TripAdvisor and just bring up the Florida Keys. There are so many things to do that you can easily fill up 4 days, probably 7-8 days, easily.
We were there the first week of June last year and it was hot and humid. Yes, temps were in the high 80's and low 90's, but with the humidity, it felt much, much warmer. You can find camping spots in June because other people have already left southern FL because of the heat and humidity. Just an FYI.