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.