Cappadocia is amazing and I would give it at least 2 days, maybe 3 if you take a balloon ride which I would recommend since you are there. There are tours of the underground that you can do too and I think your guys would be blown away by the balloon ride and the underground.
I would rent a car to do your touring; we did a tour last year, and, outside of Istanbul, driving appeared to be less stressful than I experienced in either France or Italy, and you can set your own schedule. The key is where to rent one which I can't be of help.
One another issue, I think that 5 days in Istanbul will result is rolled eyes and nasty looks from your travel partners, and 4 days would be right-this past April, we spent 3 full days in Istanbul and could have used another. WE stayed in the Doubletree Old town for €85 which was 500m from the bazaar and basically close to everything with a terrific staff. I would also suggest you look into the DK books "Istanbul top 10" and "Turkey top 10" as a supplement to the Rick Steves guides for help.
Finally, if you have more than one guy traveling with you, go to the Gallipoli monument, it's very special.