Sudden Strike 4

Sudden Strike 4 stacks with both Improved Critical base mastery and Improved Critical 8, per the normal double-triple-quadruple stacking rules.

Sudden Strike 6

The bonuses from Sudden Strike 6 apply with the weapon you drew, just like all the other bonuses from the feat. If you throw that weapon away (or at someone) and draw another, the bonuses from the first weapon do not carry over to the second weapon.