Yes. You pay for 3G data, not wireless or bluetooth internet data.
It only matters if you heavily web surf or heavily use email, but 4 GB of data is quite a lot for someone like me at least, on a tiny screen like an iPhone.
No one can answer this question for you - it depends on what you will be using it for.
How much email, email attachments, and the size of the email attachments downloaded and sent with your computer?How much web surfing is done with your computer and how much graphics do the websites that you frequent include? Do you use iTunes or any other option to download music and/or video? How often do you use YouTube to stream video? How often do you watch video with any other website? And on and on. Depending on your average use with your computer each day, 4GB in a month may be plenty or you may burn through that in less than 30 days.