A: You may or may not need either. All iPads come equipped with Wi-Fi support, so you can use them anywhere that Wi-Fi is present. For most users, 3G or 4G, which are cellular connectivity options — just like your cell phone, but without the ability to make calls — come into play when they’re not in a location with Wi-Fi.
As for the difference, 4G can be up to five times faster than 3G, which can make a big difference when you are downloading/receiving or uploading/sending large files. In both cases, in order to use the service, you purchase a set amount of data from the cellular provider each month. If you exceed that amount of data, the provider will charge you extra — and it can get expensive. Be aware of how much data you are using and how much you have left for the month!