Can I tether data from my iPhone 4 to my iPad?

Yes, if your cellular provider has a tethering pland and if you have subscribed to a tethering plan. Check for the HotSpot option on the phone. Settings - Hotspot = On

I recently set up tethering from my wife's iPhone 4 to her iPad. Quick call to Verizon, a reboot and the feature was active. Didn't work out so well. We found that establishing the tether was extremely hit or miss. The hotspot would often disappear from the iPad's Wi-Fi network list. The tether, when working, was extremely slow. Even with a decent 3G signal. It wasn't worth the additional $20/ mo. We had it disabled before we were billed again.

Also, the Verizon tethering plan is limited to 2 GB of data per month, regardless of your iPhone data plan.