Maybe I'm not understanding your situation correctly, but if you are able to run both K5 Cool and K5 Warm in your 9900, why would you want to do anything with K4??? If all your nozzles are good in your 9900, then IMHO you should be using K5 all the way (both Warm and Cool). This gives you maximum print quality from the Pro inks.
On the other hand, if you have one dead nozzle on your 9900, then (and IMHO ONLY then) you would use K5 Cool and K4 Warm. This is what I had to do on my 7900 that has a dead green channel. There is no problem at all with this setup.
The only thing that might be a bit confusing is that Walker has provided individual K5 curves for all the supported papers for the x900 printers, but apparently they ran out of paper while making the new Pro curves and have supplied only the "Master" curves for the K4 setup. Depending on what papers you use, you might be able to just use the Warm Master K4 curve, but if you also have the PPE tools you could start with the Master and linearize it for you specific paper (that's what I did). In any case, if you use the K5 Cool / K4 Warm setup, you must definitely make your own Neutral curve (using the Blender Tool if you have PPE tools). Otherwise, you could just use two curves (Cool and Warm) in QTR to setup your blends and splits.