When you want or need to be on the internet, all iPads will automatically use wireless whenever they can sniff out a wireless network. If there is no wireless internet available (car, park, travel, places that don't provide wireless), the 4G model with a data plan will automatically connect to the 4G network instead, but the not-4G model won't be able to use the internet. You have to have a data plan to use the 4G connection.
If you will usually or always use the iPad online where wireless internet is available, you don't need 4G. I will use mine at home or work with wireless access; on the occasions when wireless isn't available, I will survive without my iPad, so I don't need 4G.
You need 4G if you will want or need to use the iPad online where wireless internet is not available.
You're spending an extra $130 for the 4G-capable iPad, plus the data plan. AT&T's data plans range from $15 to $50 per month, Verizon's from $20 to $50 per month, no contract or cancellation penalties for either one.