contacting combilift is an option and they are most helpful.
but first is there another model number shown on the lift anywhere?
secondly, as long as you have verified that the direction switch is in neutral and the parking brake is set, then you should start troubleshooting the start circuit.
In general you have done most of the checks to verify the circuit except for the part of it that actually goes from the keyswitch to the control relay then to the starter solenoid.
Yes there is a relay 1 that controls the starter circuit, there is also a fuse but if the machine powers up and the dash lights up then most likely the fuse is ok as it has multiple circuits it controls including the start circuit. The relay 1 would be where i would check if all else is energized when the key is in the 'on' position.
Now based on the C4000N series there are a number of relays underneath the dash panel and the starter relay is indicated as index 1 located on the lower row of relays but would be the one on bottom left closest to the operator of the bottom row of relays. You really would need the layout diagram to know exactly which one it is but it may be numbered.
If you contact combilift they can tell you exactly what to do. The model number you gave does not tell me what the actual model number is, even in all my combi training information they do not have a decode chart that translates that number to the model number you see on the outside of the machine like CB5000 or C4000N or C5000GT etc etc. Only combi can translate that to my knowledge. This was one of the questions i had during my training that never got answered and apparently they did not feel the need to do this for some reason.
Anyway good luck
i think you'll find whatever the problem is it will be something your doing out of sequence as i eluded to in the beginning of my post. ie: parking brake not being set.