Wait, you connected your iPhone 4 to your iPad 2 by bluetooth and you can tether? That doesn't sound right.
Yes. Now, this won't work for the OP, because he doesn't want to give up unlimited data, but you can definitely do it (and if you're willing to jailbreak you don't have to give up unlimited).
Here is my setup.
* Jailbroken iPhone 4 on AT&T with 4.2.1. For the purpose of the tether, the only reason I am jailbroken is to enable tethering with TetherMe. If you have this option enabled from AT&T and can enable bluetooth tethering on your phone, you are good to go.
* NON-jailbroken (obviously) iPad 2 running stock 4.3.
It's being reported on various blogs and news sites how to do it. I listed my steps below. I should say that mine failed the first time I tried, but I have been successful ever since. So, don't give up if it doesn't work once.
1. Enable bluetooth on both devices.
1.5 Disable WiFi on the iPad.
2. Enable tethering on the iPhone.
3. Select the iPhone from the list of available Bluetooth devices on the iPad.
4. Confirm the dialog that should come up on each device that asks you to verify that the code is the same.
5. You will have a new icon in your iPad's status bar indicating a bluetooth network connection. Enjoy!
No matter if your iPhone has a wifi connection or not, this is sharing your cell data.
(Edited to add step 1.5, which I'm not sure is important, but I think made a difference in my case. Also added new icon description.)