It can mean other things too. Basically, too many T3's can lead to a T4. T3's and T4's can be caused by all kinds of things, including hardware failure. Check signal levels first, make sure they are within spec. and if you do buy a new modem, make sure you can return it incase that isn't the issue. A t4 is almost always service effecting, unlike T3's, so if you continue to have them, it may be time for a service call from your ISP.