I found them to be very similar and I didn't notice a particular cast from one to the other. There are differences, particularly when compared side by side or under a loupe looking at a solid colour.
Susnset orange is, not surprisingly, improved with the 10 colour ink set, but you do need to be looking closely. There's a similar advantage when looking at greens. Again, not surprising.
Interestingly, on my prints here I see slightly more magenta in the skin tones from the 7890 than the 4900. Neither are bad and the difference is very small. I've sent a set of prints to Mark for him to review and comment, so it will be interesting to see what he sees, too.
B&W (desaturated RGB) I'm seeing slightly darker and more contrast on the 4900 in the aforemention skin tones.
The match from the 9900 to the 4900 is extremely close (as you'd expect).
I'm keen to hear Mark's report once he receives them and has some spare time.