I'm debating between a Wifi or 4G iPad. My question is... If I buy an iPad 4G, I want to use Wifi, so does that mean I have to pay for the 4G even if I don't use it? Like let's say I'm going on vacation for a month, can I just pay for data just that one month? Or is it a contract thing like a phone, and I have to pay monthly, even if I'm not using it?