Here is the exact link for Tmobile Upgrades. http://www.t-mobile.com/bbupgrade/
There is a 4.0 OS upgrade for your device that link will take you there to download if you are not a tmobile customer here is the dummy number to put in when it requests a phone number 6156680000 and download the handheld OS upgrade. The desktop Manager 4.1 will not upgrade the OS on your bb you need to download the Handheld OS upgrade.
Hope this is helpful!