Will this bump you down to 3G speeds after the Data Cap Limit or stop all together?
Slow you down? Stop? Heck no, the'll just charge you for your overages. The faster you can rack up those charges the better.
It does say the dongle is 3G compatable but that just means that you can still use it at 3G speeds in the 98% of the country that doesn't have 4G yet.