You'll feel like you've lost a lot of control of your board when you go from softer flex to stiff flex. Going from a 4 to a 6 is pretty significant. If you haven't tried them on and tried flexing your ankles and testing the motion with the stiffer boots, you ought to try them on in the store for awhile before you buy them. And put your soft boots on before you go try the stiff flex so you'll be able to tell the difference. Typically softer boots are more freestyle and park riding while stiffer flex boots are for all mountain and high speed riding. Good luck
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