Cache is v4

Author: finsberg

.github/workflows/build_docs.yml

@@ -27,7 +27,7 @@ jobs:
 
       # preserve pip cache to speed up installation
       - name: Cache pip
-        uses: actions/cache@v5
+        uses: actions/cache@v4
         with:
           path: ~/.cache/pip
           # Look to see if there is a cache hit for the corresponding requirements file