add BETA4 to the 13.o schedule

Due to a potentionally intrusive change to address a performance
issue with UFS, add BETA4 to the 13.0 schedule to allow time for
testing.

Sponsored by:	Rubicon Communications, LLC ("Netgate")

website/content/en/releases/13.0R/schedule.adoc

@@ -40,11 +40,12 @@ Announcements regarding the availability of the ALPHA snapshots will be sent to
 |BETA1 builds begin |5 February 2021 |5 February 2021 |First beta test snapshot.
 |BETA2 builds begin |12 February 2021 |12 February 2021 |Second beta test snapshot.
 |BETA3 builds begin |19 February 2021 |19 February 2021 |Third beta test snapshot.
-|RC1 builds begin |26 February 2021 |- |First release candidate.
-|RC2 builds begin |5 March 2021 |- |Second release candidate.
-|RC3 builds begin * |12 March 2021 |- |Third release candidate.
-|RELEASE builds begin |19 March 2021 |- |{localRel}-RELEASE builds begin.
-|RELEASE announcement |23 March 2021 |- |{localRel}-RELEASE press release.
+|BETA4 builds begin |26 February 2021 |- |Fourth beta test snapshot.
+|RC1 builds begin |5 March 2021 |- |First release candidate.
+|RC2 builds begin |12 March 2021 |- |Second release candidate.
+|RC3 builds begin * |19 March 2021 |- |Third release candidate.
+|RELEASE builds begin |26 March 2021 |- |{localRel}-RELEASE builds begin.
+|RELEASE announcement |30 March 2021 |- |{localRel}-RELEASE press release.
 |Turn over to the secteam |- |- |{localBranchReleng} branch is handed over to the FreeBSD Security Officer Team in one or two weeks after the announcement.
 |===