The native iPhone4 hotspot feature is 20 bucks/month.
The iPad 3G data plan is 25 bucks for 2GB of bit consumption.
If you are grandfathered onto the 30 dollar unlimited data on your i4, the hotspot feature will kill it, and your i4 will fall to the 4GB data pro for 45 dollars.
If you have the 25 dollar 2GB data plan on your i4, you will have to migrate to the 4GB data pro option when you enable hotspot for tethering.
In your case, there is no economic ROI on the tether, since you have a 3G radio on the iPad and can get a data pipe for 25 bucks, not have to worry about managing one pot of data across 2 devices, and splitting the performance in the event you are sharing the pipe simultaneously.
