To compile GNOME 3.21.4, you can use the jhbuild [1] modulesets [2]
(which use the exact tarball versions from the official release).

[1] https://developer.gnome.org/jhbuild/
[2] https://download.gnome.org/teams/releng/3.21.4/

The release notes that describe the changes between 3.18.1 and 3.21.4
are available. Go read them to learn what's new in this release:

core - https://download.gnome.org/core/3.21/3.21.4/NEWS
apps - https://download.gnome.org/apps/3.21/3.21.4/NEWS

The GNOME 3.21.4 release is available here:

core sources - https://download.gnome.org/core/3.21/3.21.4
apps sources - https://download.gnome.org/apps/3.21/3.21.4

WARNING! WARNING! WARNING!
--------------------------

This release is a snapshot of early development code. Although it is
buildable and usable, it is primarily intended for testing and hacking
purposes. GNOME uses odd minor version numbers to indicate development status.

For more information about 3.21, the full schedule, the official
module lists and the proposed module lists, please see:

https://www.gnome.org/start/unstable

For a quick overview of the GNOME schedule, please see:
http://live.gnome.org/Schedule