Hi Mrpaul, you're the most recent owner to praise the new Def4s. Mine are to be installed in exactly 2 weeks. They've been out for a pro review, so should be no bedding in issues.
I've bought them blind (deaf?) w/out audition to replace my beloved 2s, so this unanimous thumbs up is filling me with (cautious) confidence.
The thing I am most concerned about is whether there is any loss of the seductive Zu tone dense presentation, with perhaps a move to a more spotlit hi fi type sound.
I would hate it if improved treble extension, midrange transparency and bass control in the 4s led to a hyped up exaggerated presentation I moved away from in the first place going to Zu 5 years ago.
Deep down your comments as well as Phil's and Gopher's should mean this fear is unfounded, but as in so much of audio, new very rarely means truly improved.