I voted 4, even though it is a bit over 4.
Probably 70% of the time, I am around wifi. Internet radio has it's claws in me. If I am going somewhere where I don't know the radio stations, which recently has been rather often. Streaming radio adds up quite a bit, that makes the bulk of my data use monthly. There was a period in time where I was in my car for 8+ hours a day. If I am staying in a hotel, I can't sleep in silence, or hear the outside noises. I could use the TV to mask it, but I have Pandora. So all night long she streams me lullabies. Some, actually most hotels seemed to have wifi, but sometimes I wouldn't use it, no real reasoning why though. I heard Pandora uses roughly 50MB an hour, which is why I have so high of usage. My highest month was 9GB, which was 4 months ago, so out of my average.
My web browsing is usually extremely basic, checking a few forums and news sites. I e-mail a little bit, but nothing crazy, no huge transfers. I don't really tether, not that I am against it, it is just that I rarely have a need for it. I also have a few widgets that poll for updates from time to time, but nothing major. Basically Pandora = 3+GB a month on my phone.