To echo everyone else really...consistency consistency consistency!!
I was as quick with my B4 on a lap for a single lap...but just found it really tough do it lap after lap on the B4...ended up having to hold back a touch to be consistent.
Got the X6 and because the balance so much more even...the pendulum effect Chaz mentioned, I could post my B4 fastest laps, lap after lap, without having to hold back to reduce the risk of pushing too hard.
That said, if your good and consistent with the B4 already...try an X6 and see if you can push even more...maybe it'd work out for you? Certianly never regretted to move myself, and they do suit our UK indoor and outdoor tracks really well...even on the slippy sections!
Come along to Caldicot on any Friday night, been a while since we've seen you, there are always a few running around...and Si and Dan testing their Chunks as well...as you've seen in the lastest vids!