The aspect ratio will be 4:3 for the shows until VOY, because of the same reasons TNG is in 4:3.
I asked Rick Berman on Twitter recently, whether either DS9 or Voyager were ever protected for 16:9. This is what he answered:
DS9 and Voyager were framed with 16/9 in mind...somewhere down the road. Enterprise was shot mostly digital with 16/9 a sure bet.
As LOKAI has already pointed out, Berman is mistaken in that Enterprise was shot mostly with 35mm film, not digital tape.
And when Berman says DS9 and Voyager were framed with 16:9 in mind somewhere down the road, what he really means is that every episode was framed for 4:3 and later on (sometime around '96/'97) it was decided that a 16:9 version would share the same width as the 4:3 area (so-called "common sides") and would have to be meticulously reframed on a shot-by-shot basis via a tape-to-tape Tilt & Scan online repositioning session. This is exactly what happened to the Enterprise pilot "Broken Bow" (and a few other early S1 episodes, I suspect) which was shot 4:3 (just look at the behind the scenes featurettes that show production monitors) and then reframed under the guidance of the director, James Conway, at UPN's insistence because they decided at the last second that they wanted to air the show in 16:9 letterbox.