And, if you are in the USA, there are only two types of 4G iPad3s - the one for Verizon and the one for AT&T. Although they are "unlocked," they are not compatible with each other - you can't switch plans or SIM cards.
Then, the plans offered by both carriers are for 30 days at a time; there is no contract. For AT&T, the monthly fees are 250MB/$14.99 - 3GB/$30.00 - 5GB/$50.00. For Verizon, the monthly fees are 1GB/$20 - 2GB/$30 - 5GB/$50.00. You'll have to decide which carrier is better; there is much debate...
You can buy a 30-day block and cancel it right away. After that 30 days is gone, the plan then goes inactive. So, it's not like a phone where you have to sign up for a 2-year contact. Or, if you don't cancel, it automatically renews (and will continue to renew until you cancel it). But, you're not locked in.
Hope we've cleared things up for you.
Marilyn