updated to v4 in .github/workflows

Author: BrandonWong14

.github/workflows/pull_request_ci.yml

@@ -25,7 +25,7 @@ jobs:
     steps:
       # First, we check out the repository's code using the Github Actions' "Checkout" tool.
       - name: Checkout code
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
         with:
           submodules: 'recursive'