Well, actually, "_dev" means we haven't released that version yet. So the next will be without _dev -- 13.3.4 and 13.4.4. I made the mistake last release to increase the number as well, so that's probably what is throwing you off.
13.3 is the "stable" version.
13.4 is the "development version.
Within those, we are going .1, .2, .3, etc. And a few months ago Jaime suggested I add the dates to the dev subversions. I also decided to keep it all synchronized so that 13.3.n was being produced at the same time as 13.4.n in terms of bug fixes. Now at http://chemapps.stolaf.edu/jmol/zip
we have a full record of the daily (subdaily?) changes in a flat directory. That's working very well.
So 13.3.4 and 13.4.4 will be released momentarily.