Like some of the others have mentioned, four days is not that long in Istanbul. There is ALOT of ground to cover, but if you are set on doing a day trip, I would also suggest Princes Island. A Bosphorus Cruise is a must, but if unless you prefer to be alone, don’t take the private cruises. They will cost you 30-35 lira to go the exact same places as the local ferry operators- only 2 lira. I made this mistake the first time I was there and regretted it. Have fun!