Upgrade the OS first. Once you have 4.5 installed on the device, then install JL_Cmder and and run the ResetToFactory option. That will remove the IT policy and restore your device as if it came right out of the box.
If you're looking to use the BlackBerry to its full potential, you may want to look into a BlackBerry data plan, if it's within your budget. It all depends on what your needs/wants are for the device.
That is for a later discussion. Let's stick to upgrading your OS and removing the IT policy.