I had the same problem as crailey until I finally gave WiFi Tether some tough love... While I had uninstalled and re-installed every version of Wireless Tether many times with no joy, one time I tried clearing data (via Menu / Settings / Applications / Manage applications / WiFi Tether / Clear data) BEFORE uninstalling WiFi Tether. Then after re-installing, it worked on 3G AND 4G... After that I could switch 4G on and off multiple times and it all worked. Still I sometimes I lose the connection and I have to stop/restart Wireless Tethering once or twice to get the connection working again, but it's not a major inconvenience.