The asian side has its own interesting things, and is probably worth checking out once over four days. Kadikoy is interesting and it’s worth taking a ferry over just for the amazing views and the great ride for so cheap. When you get to the asian side you can just have a balik ekmek or some midye dolmas (see food) and then hop on a ferry back to eminonu or karakoy. its a cheap ride, a great view, and pretty relaxing if the weather is nice and you sit outside. You can stay in kadikoy longer if you have time, it’s a pretty cool area itself.