That being said, it is possible to use your iPhone 4 data plan on an iPad 3G without jailbreaking. Follow these steps at your own risk:
1. Make sure 3G cellular data is off on the iPad. To turn this off go to Settings -> Cellular Data -> Cellular Data -> OFF
2. Remove the SIM card trays from each device using a SIM removal tool or a paper clip. The micro-SIM cards and trays are small so handle with care.