A 4cm threshold and another rail on top of it seems very difficult to me. The highest threshold here with us in Zurich is 2.7 cm high 3.5 cm away because of a front elevation , and that goes just fine. The Dream X50 finds its way, does not give up when overcoming obstacles and learns all the time. Because of the obstacle wheels and tipping function, it really is an asset. We used to have a predecessor modell where we had to put wedges in front of the thresholds. This is no longer necessary now. But for your threshold with rail, you may still need one more wedge quasi as a buying risk. Good luck and have fun. Mvg Marco from Zurich, in Switzerland for 33 years.