Why? The 4g network is tracking you via cell town triangulation the whole time anyway. Might as well use the phones wifi when you can save some bandwidth on the slower cellular network.
(Settings -> Privacy -> Location).
It's probably not as comfortable as the solutions of iOS/Android, but it gets the job done.
http://msdn.microsoft.com/en-us/library/windowsphone/develop... - 2.7.3