duplex: just as in
duplex apartment means there are two floors, here it means there are two voices/tracks to sing, probably a deeper one and a higher one.
a 4: in French and Italian (and probably latin ?), this is how you would say
with 4 people. For instance,
to play (a game, music, ...) with 4 people would translate to
jouer à 4 in French. So it really just means 4 people are singing.
In the end the definition of
duplex a 4 is exactly what you wrote:
there are 4 voices singing two canons in parallel.
EDIT: 4 people re singing in total if it's
a 4. Then if it's duplex you just group those people onto different voices.