You can mixdown to any thing, since you will be playing back your 4 track tape on the same unit (With DBX) it will be sent out normal.
(DBX is not compatable with Dolby) but since you will be recording on a new tape at mixdown, the unit with Dolby will play back it's own encoded tape.
I dont recommend recording on your mixdown deck with Dolby and then playing back that tape in your 4 track. It might sound real bad, but it wont hurt tape or machine!
Sincerely;
Dom Franco