How much internet does 4gb have?

I'm bringing my iphone and not sure how much gb i need

i'm thinking i'm using one through two hours of internet every other day (on facebook, tumblr) and not sure how much videos i'm going to be watching in youtube, perhaps in one month twenty videos, that range from one minute through ten.
is that enough or

(would it change much if i got 5gb instead of four too)
(oh and i'm thinking i might use t-mobile)

To me, it doesn't make a difference whether it says 4gb or 3gb or 5gb. Its all the same. The internet itself is not what affects the speed at which your phone reads the internet. It is the phone itself.

So you should be fine as long as your phone is fast.