Best of Istanbul in 4 hours

4 years ago Istanbul airport offered a free bus tour of the city but I think there is now a new airport so not sure that is still offered.

As to luggage, some airports don't allow luggage check in until 3 hours before...so ensure that is allowed. I would be asking on a destination specific travel forum with local regulars answering the questions....or review https://www.sleepinginairports.net/guides/istanbul-airport-guide.htm for help with luggage storage. I understand the security here is very tight with 2-3 checks, and as per any airport, you are supposed to be there 3 hours prior to an international flight. I hear 4 is not too many for Istanbul.
As to Blue Mosque, you need to time it around prayers, so the website will define time allowed in on your dates. If you look at that photos, that is what you see. You need to queue, if not appropriately dressed, then you need to organize coverings (they sell or rent).
There is an entry area where ablutions (washing) was done and this has some info boards to read. If lines are long then maybe 90 minutes? Taxi is fastest. Though things are located in a fairly central area in Istanbul it is a large city so 'blocks' are big. I don't recall easy food places and 'grab' isn't a Turkish practice. I think there may have been some rolling carts for hot dog type thing, but no guarantees.
6 hours doesn't really give much time to do anything. I know it is long while waiting, but given the distance and security complexity, you may want to rethink.

Only 10 to 20% of the Blue Mosque is open. The rest is under scaffolding for renovation.

I don't think you have time to visit Istanbul. The ride from the new airport is 1 to 1-1/2 hours each way, maybe even more if during rush hour traffic. Our ride into town a week ago was one hour without traffic. But getting from the gate to passport control, getting lira, getting bus ticket, and getting on bus was an additional one hour. By the way, most flights arrive late, because of the long taxiing times to the terminal.

The prayer is not a problem as I do practice my religion as much as I can. Yeah, 7 hours just sitting on the airport is too long, and would not anything good for me. I totally understand the security became a such high protocols now. My plan is take no luggage with me except some essential belongings, purse etc.

I've not done it from the new airport, but with similar times from the old Ataturk airport. I suspect you probably will not have time. It took me at least an hour, closer to an hour and a half to clear immigration. This means it is likely to be at least 7PM before you get to central Istanbul, and you'll need to be back realistically at 9pm (9:30 at the latest) - which gives you an hour maximum once you factor in the trip out.

If you had had the time, and depending where the bus dropped you off, I would have suggested a quick look at the outside of Hagia Sofia and whatever you could see under the Blue Mosque, then tram to Eminonu, get the ferry across to Kadakoy (the views of Istanbul from the water, either lit up, or at sunset are wonderful.

yes, What I searched it seems to exactly what you said, it will take me an hour or bit more than an hour to get to the city if I am very efficient with planning. Traveling from the airport to city is lot of time. I was looking at the tripadvisior about the boat tour I am not sure how worth it is. lets see. There was link sent to me for airport guide but I could not find any contact info as it is under the construction.

If you're flying Turkish Air, see if you are eligible for one of their tours (that would be the most efficient approach by far). It's not a given and really depends on arrival/departure times, but they do have such a program for long layovers as a way to promote the city.

I agree with Agnes. If you are flying Turkish Airlines they offer a free (I think) city tour for people with long layovers. Check with them for the details. Also, there is a poster named Enigma on the Tripadvisor Istanbul forum and he is very familiar with the airport and the city. He would be a good resource for you in this situation.

I would choose something else other than the Blue Mosque to see unless you are wanting to pray. As it is right now, it isn't worth the travel time. If you can get into Hagia Sophia, that would be worth it.

wow! that is most helpful thing to learn. Yes, I am flying with Turkish airline and I would check with them now. Yes, I will definitely look for the other resource as you suggested. thank you so much!!!!!!!!!!!!!!!

Unfortunately, the tour schedule looks too tight given your arrival and departure (https://www.turkishairlines.com/en-us/flights/fly-different/touristanbul/tour-schedule/index.html). Maybe you can see if you can change your schedule a bit to accommodate more time in Istanbul? I'm not sure of what Turkish Air's change ticket policy is, and whether it would be worth it. You're right in that awful zone of having too short of a layover to really make use of it. If it were me, I would enjoy the new airport. I heard that it's huge and I'm sure offers many amenities (massage, shopping, food, etc.).

The tourism service from Turkey airline responded below. They start the tour at 6p but it depends. As you mentioned the layover is very tight.
TOURISTANBUL is a complementary free service for TURKISH AIRLINES‘ international transit passengers. (Who arrives from an international destination and travels to another international destination within , 6hrs - 24hrs) The carrier must be TURKISH AIRLINES for the connecting flights. While forming the itinerary passengers can select two connecting flights with a long layover in order to join TOURISTANBUL. The tours can not be joined after departure

We kindly remind you that our tours are being guided only in ENGLISH

Tour hours from Istanbul Airport:
08.00-11.30 / 08.30-15.00 / 08.30-18.00 / 12.00-18.00 / 16.00-21.30 /18:00 -23:00
These are the arrival and the departure times of the tour bus from/to the airport

We do not have any booking or listing. We kindly ask you to proceed to TOURISTANBUL Desk after passport and custom control half an hour before tour starts. (Touristanbul desk is the opposite of the meeting point 4 at Arrival Flour)

Tourİstanbul is definitely free of charge for our international transit passengers(Guiding servise with professional tour guide,meal (breakfast and /or lunch depends on tour programmes),transportation with shuttle bus and all museum fees are included.)

Although it may seem like a long connecting flight, I wouldn't risk it. The new airport is huge and there are a lot of things to do inside. I am worried with all the crazy traffic jam in Istanbul, that you might not get to see any sights and be simply rushed. You have to be at the airport 2 hours before your flight, so that leaves you with 5 hours out in Istanbul. By the time you get out of the airport, and get to Istanbul it will definitely take and hour or so. You could easily take a cab, but make sure he taxi driver speak English, otherwise you are going to be late for your next flight. Hope everything will be OK!

I am not sure what it is like at the new airport but when I landed in Istanbul last November at around 4:50 p.m. the border and customs line was very very very long! In all of my years of traveling I had never seen something like that. I was in that line for about 2 hours. That being said, there was a fast lane for passengers traveling business class on Turkish Airlines - However, on the way back I used the fast lane because I used a transfer service from my hotel and fast track was included in the fare. ( It is also offered at arrival). So, If you aren't in business class look at a transportation provider that might offer the fast track. If not, I would not risk it. Lastly, traffic in Istanbul is a pain ... a 5 mile drive can easily take 30-40 minutes depending on the time of day.

The lines at the new airport are not long (IME).

Once you get to Istanbul City Center,you can go to the main mosque, and catacombs, both of them are located not that far from each other. Also, like many have suggested, you can use the free tourist shuttle provided by TurkishAirlines. Personally, my favourite in the historical city center was an underground trolley ride. That trolley takes you to Galata Tower area (I might be obsessed with trolleys though).
Please be aware that the airport has moved to the new area! It will take you about 80 minutes to get to the historical city center by car. The city hasn't built public transportation line (metrobus/train) that can take you directly to the new airport from the city center. So please make sure that you have enough time for all of your adventures!