Speakers with no rear wall, better or worse sound?

Different. Maybe better. 4 feet is too close to be optimum for many speakers - 5 feet, 6 feet, more is often better.

You'll push the front-wall reflections out in time which helps sound-stage depth, drop the front wall reflection intensity which may improve timbre because frequency response isn't as uniform at extreme off-axis angles, have less bass (which can be good or bad depending on how the speaker's baffle step was designed to compensate for wall proximity), move the SBIR null lower in frequency and diminish it (good), and energize room modes differently (which can be good or bad depending on where the speakers and listener rare).

You want the speakers you mention at least 5 feet out from the wall. The more the better. Mine are 7.5 feet and the sound is glorious. I measure from the front of the speaker. Perhaps 13 is too much, but I would have to hear them. The rule of 1/3's works very well based on my experience and current set up. My room is 24 feet long and my speakers are 7.5 feet out from the wall.

I bet 8-10 feet would be ideal for you. You should still have body without bass overhang and great imaging a focus.

Putting up a wall behind your speakers will lessen the perceived depth. Whether it's a visual or acoustical thing I don't know, but you will notice an amazing reduction and probably not like it at all.

Best soundstage depth I heard out of my system, I placed my speakers a third of the way into my 13x18 room (on the short wall) with a wide, open double doorway behind them, a 10x13 entryway beyond that, and beyond that through another open set of double doors, the living room. The distance from the speakers to the nearest full rear wall was 36 feet. Despite the speakers being closer to side walls, I do think I was hearing the depth of the soundstage on each recording more accurately than I ever had. Unfortunately, bass was MIA (lacking body and depth) and the placement was a bit awkward to live with.

I'd imagine you're probably getting most of the benefit already with a 14 foot distance? You could try some diffusion on the back wall--like a jungle's worth of plants. They help a lot.

Ablang

I really can identify with your situation which sounds quite similar to my set up. We also have a double door opening (French doors removed) 3 feet behind the speakers with about a 3 foot wall section flanking the double doorway opening on either side. There is another room behind the speakers with the back wall mostly glass. The depth of the soundstage is way back into the second room and bass is not all that bad to my ears. I have been speculating that the combination of a partial wall with the large double door opening and lengthy distance to the rear wall might be an 'ideal' mix of minimizing reflected sound behind the speakers without losing too much 'body' or room ambiance.