Update changelog

CHANGES.md

@@ -28,6 +28,16 @@ creating a new release entry be sure to copy & paste the span tag with the
 `actions:bind` attribute, which is used by a regex to find the text to be
 updated. Only the first match gets replaced, so it's fine to leave the old
 ones in. -->
+-------------------------------------------------------------------------------
+## __cylc-8.0rc4 (<span actions:bind='release-date'>Pending</span>)__
+
+Fourth Release Candidate for Cylc 8 suitable for acceptance testing.
+
+### Enhancements
+
+[#4877](https://github.com/cylc/cylc-flow/pull/4877) - Upgrade the version of
+Jinja2 used by Cylc from 2.11 to 3.0.
+
 -------------------------------------------------------------------------------
 ## __cylc-8.0rc3 (<span actions:bind='release-date'>Pending</span>)__