Update version string and add warning on import.

Author: twiecki

pymc3/__init__.py

@@ -1,4 +1,4 @@
-#   Copyright 2020 The PyMC Developers
+#   Copyright 2021 The PyMC Developers
 #
 #   Licensed under the Apache License, Version 2.0 (the "License");
 #   you may not use this file except in compliance with the License.
@@ -13,7 +13,7 @@
 #   limitations under the License.
 
 # pylint: disable=wildcard-import
-__version__ = "3.11.1"
+__version__ = "4.0"
 
 import logging
 import multiprocessing as mp
@@ -27,6 +27,10 @@
         handler = logging.StreamHandler()
         _log.addHandler(handler)
 
+_log.info(
+    "You are running the v4 development version of PyMC3 which currently still lacks key features. You probably want to use the stable v3 instead which you can either install via conda or find on the v3 GitHub branch: https://github.com/pymc-devs/pymc3/tree/v3"
+)
+
 
 def __set_compiler_flags():
     # Workarounds for Aesara compiler problems on various platforms