Update README.md and use v4 of checkout action (#1437)

Update examples to use latest available checkout action v4.

README.md

@@ -91,7 +91,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
 
     - name: Cache Primes
       id: cache-primes
@@ -122,7 +122,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
 
     - name: Restore cached Primes
       id: cache-primes-restore
@@ -229,7 +229,7 @@ Example:
 
 ```yaml
 steps:
-  - uses: actions/checkout@v3
+  - uses: actions/checkout@v4
 
   - uses: actions/cache@v4
     id: cache
@@ -259,7 +259,7 @@ jobs:
   build-linux:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
 
       - name: Cache Primes
         id: cache-primes
@@ -286,7 +286,7 @@ jobs:
   build-windows:
     runs-on: windows-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
 
       - name: Cache Primes
         id: cache-primes