I tried to look up dimensions on the D9400, but WG didn't have anything for that model. If you can tell me the width, I can tell you whether or not it will fit. I would imagine if it does fit, you would have to do a bit of retrofitting anyways.
One thing though, if you want to get this monitor, and it won't fit in the K4p cabinet, I would really suggest you change your mind about the monitor. This cab is about as wide as you want to go, perhaps even too wide to get through most doors.