Did the retract, extend, tension dance after powering up M4. Then clicked on the home button which was predefined to bottom left of the work area. The M4 continued past this point with all belts performing with tension etc. by the time I got to the M4 the top right belt had reached its limit and it broke the
spool where the belt connects.
I have rebuilt arm (I had a spare spool available) and will test today.