2.99.4 was bumped in August 2014, see commit: https://github.com/mono/gtk-sharp/commit/9a72bb67fff7e4845b7bb430a608282668c3e4da The current version of gtk-sharp3 is still 2.99.3 in rawhide. It seems that 2.99.4 is missing from http://ftp.acc.umu.se/pub/gnome/sources/gtk-sharp/2.99/ Perhaps the source maintainer should be notified about this? I'm not sure who maintains these tarballs.
I wonder if that commit was just to make sure that git master is not saying that it is the release 2.99.3. That is how they do it on the mono git as well. Looking at the releases: https://github.com/mono/gtk-sharp/releases there is no release for 2.99.4 yet. You could as well contact https://github.com/bl8, Bertrand Lorentz. He did the 2.99.3 release according to Github.
The official last release is 2.99.3. The project is monitoring with Anitya https://release-monitoring.org/project/6378/ As Thimotheus say 2.99.4 is a in develop version.