I just got my Moto Navigation Dock yesterday and am loving it. I can confirm that the dock itself does not change your audio output. It is the special usb/audio cable that changes that. If you use a normal USB cable instead of the hybrid one, the speaker on the Droid 4 still sounds.
I just went out to my car and tried this with my Droid 2 charger and the Droid 4 dock.
Also, beware because the Verizon branded Navigation Dock kit does not include any USB cable or vehicle charger. The Motorola branded one does include the hybrid cable and the dual port vehicle charger.
On a side note, my only disappointment with the dock is that the clip at the top is engaged the screen is secured from flipping open. This is unlike all the previous Moto Droid chargers, which all allowed access to the physical keyboard while docked.
If you have any other questions or want me to try out any combinations, let me know!