I have run up to 48+6=54 inches of HD leads to a digital servo using 5 cells and a JR 940 or 955 Rx with no problems. Just use HD quality ext and minimise connections. Try to make ONE ext from the servo to the Rx. Use Gold plated contacts for sure. Test your set up with 4 cell and five cells. If the 4 cell fails, you may need amps in the ext.