4GB is not enough to switch to tethered as a primary home internet service... I have u-verse at home and even with their new data caps that caps at like 150 gb (and that's the first time I've had a cap on my home internet in ages)... not that I exceed that or anything but hardly think I could get by on 4 GB per month.
That's the one thing I'm envious of my sprint friends, they can still tether and not need a home hookup. OTOH if I call them their data drops since it can't handle voice too. (they prefer I call them on QIK video since it is data)...