I spend a lot of time at Road America which is a very high speed track with 3 long straights. I use 3rd, 4th, 5th and 6th. Based on the best locations to shift, S4 works fine for me. There are some areas, like the infamous "kink", where you don't want to shift in the middle of the turn. The cost of failure is just to high. S6 justs makes failure that much more likely.
As for S4 vs. S6 at RA, S4 works fine for me. I find it very smooth. I'm sure there are some differences from car to car. I paid $7K to replace an SMG II transmission in my 2002 M3. There are some DCT failures out there. I don't want to be one of them and repeat that experience.
BTW I really don't expect Bernie Ecclestone to call me one way or the other.