Not much for now since Debian already ships https://packages.debian.org/buster/xfce4 version
Debian freezes and maintains versions during release cycles for any package of a Debian release.
We will likely port to the next version of XFCE when porting Whonix from its current base distribution Debian
buster to the upcoming/next release of Debian (codename
bullseye) which might be in ~ 2 years from now.
Users are free to upgrade XFCE or any software by themselves at any time before that.
(As per https://www.whonix.org/wiki/Install_Software)
“Same way you would do that on Debian.”
(As per https://www.whonix.org/wiki/Free_Support_Principle)
That would actually be very useful if someone tested that so that newer XFCE versions do not introduce bugs which are blockers for Whonix. Because by the time we port, it might be to late to report bugs and see them fixed in the next stable of Debian.