Can a Verizon iPad (4th Gen) be used on AT&T GSM/HSPA+?
Yes, you can. It's unlocked, and you can move to any carrier you wish to. Of course, if you're planning to move off of Verizon, you would have to consider the terms dictated by your contract with Verizon.

Read more on Are iPad Models Unlocked? - iPad Q&amp;A (from EveryiPad.com). Here's a relevant quote:


  However, both of the iPad 4th Gen models with cellular data capability are sold "unlocked" and are capable of running on any compatible network. 


Also, a footnote Apple's LTE page states (with emphasis here):


  2. To identify your iPad model number, see http://support.apple.com/kb/HT5452. iPad models are unlocked and may support LTE networks outside the country of purchase when using a valid SIM from a supported carrier. Contact your carrier for more details.