ci: run workflows on the v4 branch, not just master

The v4 LTS maintenance branch received no CI because both workflows triggered
only on master (a stale snapshot from when v4 was the default branch). Add v4 to
the push and pull_request branch filters so v4 PRs and pushes are tested.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: vishr

.github/workflows/checks.yml

@@ -4,9 +4,11 @@ on:
   push:
     branches:
       - master
+      - v4
   pull_request:
     branches:
       - master
+      - v4
   workflow_dispatch:
 
 permissions:

.github/workflows/echo.yml

@@ -4,9 +4,11 @@ on:
   push:
     branches:
       - master
+      - v4
   pull_request:
     branches:
       - master
+      - v4
   workflow_dispatch:
 
 permissions: