If you mean the foot controller, then the T/FC-322 is a substitute that should work. It's listed as a replacement here:
http://www.sewingmachinesplus.com/234522001.php . The manual for the NX400 gives the foot controller as XC1154-051 . Brother changes the part number often but the wiring remains the same. Another substitute controller is available here:
http://www.sewingpartsonline.com/foot-control-w-cord-brother-xc6651121.aspx .
The F04 message may mean there is an internal failure. This probably needs to be fixed by a Brother authorized repair person.
I wish that I had better news for you.
Cindy Wells
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