diff --git a/HISTORY.rst b/HISTORY.rst
index 975ddbb3..715235b2 100644
--- a/HISTORY.rst
+++ b/HISTORY.rst
@@ -6,6 +6,11 @@ Release Notes
 PlatformIO 4.0
 --------------
 
+4.0.3 (2019-??-??)
+~~~~~~~~~~~~~~~~~~
+
+* Remove ProjectConfig cache when "platformio.ini" was modified outside
+
 4.0.2 (2019-08-23)
 ~~~~~~~~~~~~~~~~~~
 
diff --git a/platformio/__init__.py b/platformio/__init__.py
index ee53de46..300ff539 100644
--- a/platformio/__init__.py
+++ b/platformio/__init__.py
@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-VERSION = (4, 0, 2)
+VERSION = (4, 0, "3a1")
 __version__ = ".".join([str(s) for s in VERSION])
 
 __title__ = "platformio"
diff --git a/platformio/commands/init.py b/platformio/commands/init.py
index 64b5465d..e23d24e7 100644
--- a/platformio/commands/init.py
+++ b/platformio/commands/init.py
@@ -388,7 +388,6 @@ def fill_project_envs(ctx, project_dir, board_ids, project_option, env_prefix,
 
     if modified:
         config.save()
-        config.reset_instances()
 
 
 def _install_dependent_platforms(ctx, platforms):
diff --git a/platformio/project/config.py b/platformio/project/config.py
index e98ace55..9a23d494 100644
--- a/platformio/project/config.py
+++ b/platformio/project/config.py
@@ -16,7 +16,7 @@ import glob
 import json
 import os
 import re
-from os.path import expanduser, isfile
+from os.path import expanduser, getmtime, isfile
 
 import click
 
@@ -72,13 +72,14 @@ class ProjectConfig(object):
 
     @staticmethod
     def get_instance(path):
-        if path not in ProjectConfig._instances:
-            ProjectConfig._instances[path] = ProjectConfig(path)
-        return ProjectConfig._instances[path]
-
-    @staticmethod
-    def reset_instances():
-        ProjectConfig._instances = {}
+        mtime = getmtime(path) if isfile(path) else 0
+        instance = ProjectConfig._instances.get(path)
+        if instance and instance["mtime"] != mtime:
+            instance = None
+        if not instance:
+            instance = {"mtime": mtime, "config": ProjectConfig(path)}
+            ProjectConfig._instances[path] = instance
+        return instance["config"]
 
     def __init__(self, path, parse_extra=True, expand_interpolations=True):
         self.path = path