I have a n3 unit but want I want to use the n4 navi disc.. if i do; will the disc reprogram the entire unit n disable the unit's ability to view dvd's while the car is in motion. ... in other words i can watch dvds on my n3 unit now while driving if i upgrade the n3 unit with the n4 navi disc will it still have that feature to watch movies or will it reset.

i dont believe the n4 disc will work at all. it not the same as the n1-n3.

You need the CNDV70 disks, not the N4 disks. It will not change your ability to watch movies and crash into stuff