If you look at the Frequently Asked Questions page using the link posted above you will see that Bruce addresses the reason for the improved tail design. I did a quick search thru RCU for posts discussing this in some detail but didn't come up with any- but I know they are out there 'cause I remember seeing several about a year ago. It seems to me that part of the cause is actually the vertical position of the stab in relation to the wing, but I don't trust my memory that much.

Anyway rudder performance on the 4* is not a real issue unless you are absolutely determined to make coordinated aileron/rudder turns. The rudder as designed is fine for both ground steering and basic acrobatics. And many floks have successfully built bigger ones to get improved response. If you want a bigger rudder then go for it. I'm sure the plane will fly just fine.