What is 4G versus 3G?

I always hear about 4G versus 3G and I get the basic understanding, 4G is better and I assume provides better service. But what does this really do for me? Do I really need to have a phone that gets 4G? Right now, I get 3G and everything works just fine. Do I need to worry that in a year or two I will need 4G? Thanks.
