Yes, if you really want it to be self-service. You will need a Micro-SIM to SIM adapter. Once you have the dumbphone active, just call AT&T Customer and ask for the iPhone data plan to be removed, you can't do it self-serve.
The rep may also ask for the IMEI of your dumbphone to update the IMEI on the account when they go to remove the iPhone data plan.
Also if your long range plan is to switch back to your iPhone 4 and just use it without a data plan, don't bother as your iPhone 4 will turn you in the first time it connects to the network. 😱
Dave