Document v4 support in SECURITY file

Author: ascopes

SECURITY.md

@@ -7,8 +7,9 @@ significant impact. However, should an issue be identified, I
 will strive to fix these issues as soon as possible.
 
 Fixes will be released as a new version, built from the most
-recent `main` branch. Unless expressly requested, no attempt
-to backport changes will be made.
+recent `main` branch for Java 17, and from the v4 branch for
+Java 11 backwards compatibility. Unless expressly requested,
+no attempt to backport changes will be made.
 
 Given this library is designed for use in test packs, the main
 design assumption is that the software is being used by a