add note

Author: sentrivana

tox.ini

@@ -647,6 +647,7 @@ deps =
     spark-v3.1: pyspark~=3.1.0
     spark-v3.3: pyspark~=3.3.0
     spark-v3.5: pyspark~=3.5.0
+    # TODO: update to ~=4.0.0 once stable is out
     spark-v4.0: pyspark==4.0.0.dev2
     spark-latest: pyspark