xfce 4.13.x are development releases geared towards xfce-4.14 stable release. Xfce is a
"release when ready" DE. The roadmap [1] specifies what the status of each
sub-project is but does not give definitive dates of release.
Having said that, 4.13 is, in my experience, quite stable. They are labeled development
releases but changes in Xfce do not tend to be drastic and hence my proposal to update.