Go for a Booster strap it'll stiffen your boot up. Although it's worth biting that Boosters give a more progressive flex to the boot, so it night feel a little different at first. Most people are fine with the regular Booster, but if you want a stiffer more dynamic boot there is also a World Cup model strap too. If the 4 flex is feeling too soft, I'd look at going up to the 8 flex as a minimum. Lots of guys still ride an 8 flex in the park with no problem, because in comparison to other boots on the marker, an 8 flex is relatively soft.
My UK/EURO BST Thread - http://www.newschoolers.com/ns/cult/forumthread/thread_id/743385/
Check my blog for updates and reviews: http://aslansskiblog.blogspot.co.uk/
Joey -Formerly *III.JVD.III*
I post from mobile the majority of the time, so if the above is incomprehensible, I apologize.