The error 4 message indicates it is seeing an internal error in one of the circuits. Unfortunately it will not specify which one.There is one possibility the customer could check:On some units the 5 Volt supply drifts over time and ends up well below 5V. this can be checked easily once the top cover is removed. Attached picture shows the power supply pcb board with main power supply and other components. (in this case with rear panel removed) The orange connector at the top, right next to the main power supply, should carry 5V. if that is significantly different, more than 0.1 V down, then it will cause intermittent operation of the unit. If that is the case there are two possible fixes:-replace the power supply with an identical unit. The main power supply is a standard of the shelf power supply which may be available locally. (the manufacturers model and data are visible once it has been removed from the unit and opened)(check 5V supply before fitting it!)-reset the 5 V: the issue with the power supply is often the potentiometer that sets the 5V. this can be reset to correct voltage by removing the main power supply from the unit. Powering it up outside the LDS-6 and reset the 5 V line to the correct voltage. Then refit and recheck the voltage. If it is not possible to set a stable, accurate 5V then the supply will have to be replaced. If this is not the underlying cause of the problem then the unit will have to be returned for service