Do you want to stay in the same area (Sultanahmet) as the Four Seasons, which is central in terms of being near a lot of the sights in the old city, or do you mean central-central (eg around Taxim Square)? Do you want to stay in something that is more like a charming Turkish guesthouse (but which might lack some amenities) or do you want to stay in a more western-style hotel or boutique?

Are you going to be in Istanbul in mid-November? If so, keep in mind that the World Congress of Accountants will be there (and will be booking up a lot of the mid-range, Western-style hotels). Actually, its accommodation list could give you some ideas of places to stay.

I've stayed at the Taxim Hill hotel a few times on business (not to be confused with Taxim Square hotel beside it). It's nothing fancy, but it is relatively new with streamlined modern (relatively small) rooms. http://www.taximhill.com/ It's very centrally located and has a decent in-house restaurant. The price is about 100 euros, including breakfast and taxes. The only downside is that I think they don't have non-smoking rooms.

Another hotel nearby is the Point Hotel. It's more style-conscious and has a great view from its breakfast room. I found it a bit noisy and not the best design for business (inadequate desk), but overall I liked it. It's a little more expensive, but still a decent price - around 140 US including taxes.

http://www.pointhotel.com/