Add testing of django 4 back

then we test django 3, 4 and 5 with python 3.8 and 3.12 on both postgres
and mysql

.github/workflows/ci.yml

@@ -9,13 +9,11 @@ jobs:
     strategy:
       matrix:
         python-version: ["3.8", "3.12"]
-        django-version: ["3.2.25", "5.1.4"]
+        django-version: ["3.2.25", "4.2.17", "5.1.4"]
         database-engine: ["postgres", "mysql"]
         exclude:
           - python-version: 3.8
             django-version: 5.1.4
-          - python-version: 3.9
-            django-version: 5.1.4
 
     services:
       postgres: