Seems about right.
I'm rocking #10 on my First Chairs, and they're the stiffest flex I've felt in a boot. I love it.
As said though, all companies seem to differ from here to there when it comes down to flex of their boots.
Just curious, are you ordering new tongues? I'm sure where ever you got your FTs, you'd be able to go in and flex a #6 or #4.