Your logic is flawed. Your plan won't work.
The choices are this:
1. Upgrade at your renewal time and sign up for a tiered (shared) data plan and have them subsidize $400 of your next iPhone.
2. When the next iPhone comes out, go to apple (or wherever) and pay full retail price. Activate the phone on Verizon. (EDIT: They may actually force you to buy an iPhone from them at full price to retain your unlimited data. They may not want you to get it from apple. just a thought, nothing concrete).
If you go out and buy a used 4G device right now to activate, it will have zero impact on either of the above two options.
Let's say you go buy a 4G device right now and then proceed with option 1 above. You would still be forced onto a tiered plan if you indend to have Verizon subsidize your next phone purchase.
Let's say you go buy a 4G device right now and then proceed with option 2 above. You will still be paying full retail price for the next iPhone in order to keep your unlimited data.