It should work for data, won't be able to send ro receive calls.
I had used Data only SIM on my 3GS many times, i don't see why it shouldn't work with a MicroSIM (from the iPad) on the iP4. You may need to modify the APNs but it should be easy enough.
However I am curious, why would you want to do such a thing?
If you get an iPhone 4 you have to have a data plan, so no work around there, so if you have an iPad with data and then get an iPhone you will have 2 data plans no matter what.
Usually people try to put the IP4 SIM on the iPad since it can save them 1 data plan.
Just wondering, you maybe got really good reason to do this.