4G meaning HSPA+ or LTE? On your iPad if it says 4G its really 3G it should say LTE to get the real high speed.
From what I have read if you are grandfathered into a iPad unlimited plan you will have it for new iPad and get LTE. If you only have 4G as your symbol I would call or go to the store to get LTE working, it might just work with your old SIM but I just upgraded from a 3G/4G phone to a LTE from ATT and they said they needed to activate a new SIM since the old one doesn't work with the new LTE devices.
Anyways I've read that they are unliminted if you are grandfathered into a Unlimited iPad Data Plan. But don't be supprised if in the future they start trottling your service, happed to me went from iPhone unliminted plan to a Samsung LTE device and after 5GB of data they started trottling speed to half a meg download.