Then get the bigger board...
You can always adjust to a slightly longer board in the park, and not really feel it.
But a board that's to short in powder, takes you away from the fun prematurely
If you're really hard on your gear and think you'll want a new board pretty quick, go with the shorter one.
There's a lot of ways to look at it, and it's only 4cm. I would rather be 4cm to long than 4cm to short when it comes to my snowboard
"When I do good, I feel good; when I do bad, I feel bad. Thatís my religion." - Abraham Lincoln
Moral of the story: If your going to rob people, wear a diaper.