Your first and best option is to go to the settings -> About Phone -> Phone Updates and get any updates your provider has for the phone.
Applying an update to a later version of Android not put together by your provider is a very dicey thing. The two most likely outcomes are making your phone so it can no longer connect for one reason or another or, bricking your phone altogether from which there may well not be any means to recover.
The best I can suggest is to look at the work being done by the Cygqanomod folks. You might search for your phone model and cyganomod or some other variants such as your phone and "Android 4" ... But don't hold your breath. If it isn't a common phone or if most people simply upgraded to a newer device then you probably won't find much. Basically it needs to be a device worth keeping instead of upgrading that makes it worthwhile for the techies that own them to look at figuring out how to make the new version work on it.