The closest you can get is combining pay as you go with a data plan. However, you'd have to buy the phone at the full price, since you're not getting a contract.
No, they dont have that option available on a subsidized i4. You have to have voice.
Or pay full price for one and maybe use an ipad sim for data only.
There *is* a mic on the iPod Touch 4G and wifi is everywhere. It's making less and less sense to have a contract for a feature (voice) that's hardly used. If voice is essential everywhere use the Touch in tandem with some dumb phone.