I'm always cruising around in 5th at 35-40mph (ACH) Getting onto the highway I'm usually shifting out of 4th around 60-65 though. I think the fastest I've gone in 4th is around 70mph. With no governor or tach I'm scared to take it much faster in 4th...
4th for me pushes out to about 130 km/h and she's got nothing left.