Think of it like walking. 3G is like walking at 3 mph, 4G is like walking at 4 mph. This isn't saying how much you can carry, just how fast. Yes, in 10 hours walking at 4 mph you can make more tripsand end up moving more stuff than at 3 mph, but in the end you pay for how much you transport, not how fast you transport it. This is referred to how much data you use and the "G" plans are referred to as data plans. When we got an iPhone with AT&T we had the option of the base package with 0.2 GB data/month, the 2 GB package, and the unlimited. We got the 2 GB package and never come close to it. If we downloaded movies we'd exceed it with 4 movies easily.
Yes, it allows Internet access when you don't have wireless. However, the access may be limited to some items. Netflix would gobble up data. I don't know if you could even use it over a data plan. They restrict some of that heavy duty use to proper wi-fi. For example, you cannot Facetime with 3G, you have to do that with standard wi-fi. Data plans are a convenience but not a true replacement for WiFi. (In case somebody reads this, I am not going to go into hotspot use right now.)
There's also cost considerations. I haven't calculated it but I bet if I started downloading at the maximum rate for which we pay I could probably download over 100 GB per month. We pay $14 for our Internet service with 100 GB potential. We pay $20 (I think) for the 2 GB data plan with AT&T.