I've read while the resolution is the same between the 4.0 and 5.0, the 4.0 is the better of the two (easier on the eyes and clearer). I wouldn't know cause BB doesn't have the 4.0 in stock or on display to compare. I've been missing with the 5.0 in the store for the last week but still haven't pulled the trigger.
Of those two, I really rather have the Icer Itonic A100 for $259 currently at BB but no store within 100 miles of me has any of those. And I already have the KF btw.