Can I get an iPhone 4s for Tmobile?

I want an iPhone but T-mobile doesn't have them. If I get an unlocked one could I use it for T-mobile, and would still use the T-mobile 4g service or would it go slower?

T-Mobile is the only major carrier in the US to not offer Apple's ubiquitous iPhone, but that doesn't mean that it ignores customers that wish to use the popular device. In fact, the carrier already offers micro-SIM cards and technical support to customers with iPhones and in a blog post today, T-Mobile details how it is ramping up its efforts to cater to more iPhone customers with better support, training, and services.

T-Mobile has long said that more than one million of its customers are already using iPhones, but it seems that the carrier would like to increase that figure - even though it still doesn't sell the phone directly. Starting on September 12th (coincidentally, the same day as Apple's big launch event), T-Mobile retail staff will be trained on setting up iPhones for customers and each store will have iPhone 4S demo units on hand. The carrier is also developing iOS versions of its popular mobile apps, such as T-Mobile myAccount, T-Mobile Visual Voicemail, and T-Mobile TV, which will soon be available to download in the iTunes App Store.
http://www.theverge.com/...ed-support