Comcast says hi: http://customer.xfinity.com/help-and-support/internet/data-u...
AT&T says hi: https://www.att.com/esupport/internet/usage.jsp
However, even uncapped internet normally has a AUP (acceptable usage policy, also soft cap) that is usually around 200GB - exceed it and your internet is de-prioritized to the point of being almost useless.
Either way, 4GB favicons are probably unhealthy for the overall internet.
The contractual limits give the company a legal out to deal with use that starts to impact other customers, they are not commonly enforced unless they are looking into a complaint to slow internet and your usage comes up as anomalous and potentially contributory (according to a friend at an ISP at least).
Download or upload 2TB via torrent? - No one cares
A friend of mine who works at Swisscom (no. 1 provider here) said they were forced by their software to enter an upper limit, so they just entered the max allowed length of that integer number.
Looking back, the funniest thing about it was that I ended up having an argument with one of the support people on a broadband forum and they were arguing that only pirates would ever need more than 60gb and that internet TV was never going to take off because the BBC iPlayer trial hadn't worked well in their browser.
These days there are UK ISPs with their own 4K TV channels.