Ahmet's apartment is very close to Taksim Square and it has all facilities to accommodate 4 people. It's in an old building but Ahmet refurbishet it very nice, keeping the old touch - the brick walls and ceiling - and adding modern furniture and equipment.
Some small shops are very close to his apartment, one of them is open 24 hours, so very easy to get basic stuff anytime.
We had a very good time in Istanbul and Ahmet's apartment was our frienly base there.