The
D4 connector is a screw-on type connector with a 2.0 mm diameter
ferrule. The D4 has a key to keep opposing ferrule ends
from rubbing together while mating. The D4 has gained popularity
because of NEC. NEC specified the D4 connector on all
of their equipment interfaces. The connector's small size
make it convenient to load many connectors in a fiber distribution
frame.