bump PET release version

Author: eleanorjboyd

build/azure-pipeline.stable.yml

@@ -112,7 +112,7 @@ extends:
           project: 'Monaco'
           definition: 593
           buildVersionToDownload: 'latestFromBranch'
-          branchName: 'refs/heads/release/2026.2'
+          branchName: 'refs/heads/release/2026.4'
           targetPath: '$(Build.SourcesDirectory)/python-env-tools/bin'
           artifactName: 'bin-$(buildTarget)'
           itemPattern: |