You are correct that an error 4000 is an axis drive fault. The red LED on each drive is lit when the drive is off or there is a fault. If a drive sees a problem with a motor, or cable, or itself, it will create an axis fault and all will drop out. The whine from the chokes will cease as well. What you need is to invest in a maintenance manual (machinemanuals.net). See what is in tolerance: drive power supply, logic power supply, etc.You have 3 axis. Compare them. Measure the armature resistance of each motor. current draw, etc.
It is common to remove an encoder connection and then tweak the offset pot to balance that drive so there is no drift.
The maintenance manual will tell you how to access the FMDC board (through K3 connector) with a PC to get to diagnostics (called a fist event monitor).
Swap drives (they slide out) and see if the fault follows the drive.