Please confirm - is this error 4 or error 40?
Ok, the error 4 doesn't seem to exist as an error code on the 954i.
However the Stop Key message indicates a problem with the emergency stop safety key - have you checked that it is fitted correctly and securely, and isn't loose?
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First, you could try bypassing the stop key - if the machine works ok then this confirms that it has a fault and needs to be replaced.
You can purchase the part from www.sportsmith.net
The stop key can be bypassed by shorting out the switch wires.