If their system flags your account for suspecting tethering without a tethering plan, they send you a text stating something to the effect of "We have detected unauthorized use of tethering services. If this is in error, please contact us. If we do not her from you within seven days this account will be automatically upgraded to include tethering for your convenience."
So they send you a polite warning and will automatically swap it if they do not hear back from you in a timely fashion. Watching while on week-long business trips I have used some 15gb in a month before. I think they look for indications that someone is using it for their only internet, which can easily exceed 50gb, or if you download large multiple files at a time such as a game or video file onto a laptop. The 4GB probably wouldnt flag anything, the flag would probably come from if it were a file that would obviously be downloaded to a laptop, like a large zip file or some other non-iOS file.