Yeah, I'd make sure to ask a question on that. The rear rim for an F4i looks identical to the 929/954 rim but it is 5.5" not 6.0"
I'd make sure to just ask, "is this the 5.5" rear or 6.0" rear?" If you phrase it like, "I need the 6.0" rear to fit my 954," he'll probably tell you what you want to hear no matter what the truth is. If you say you're looking for the 5.5" he'll probably say it's the 5.5" if you tell him you are looking for the 6.0" he'll probably tell you it is the 6.0".