qt4 toolkit plugin not found when using pyinstaller

I'm not sure if this would fit better under pyinstaller or traitsui, but essentially when I try to build an exe using pyinstaller the resultant exe file raises an exception:

`RuntimeError: No traitsui.toolkits plugin found for toolkit qt4`

The first two lines of my program are:
```
from traits.etsconfig.api import ETSConfig
ETSConfig.toolkit = 'qt4'
```

The program works fine when run in a python environment, so think I just need to add a hidden-import, but I can't find what file it would be looking for since there is no toolkits folder or toolkits.py file in the traitsui folder, but there is a qt4 folder. It just doesn't seem to be looking there.

For reference, I am attempting to import mayavi but it always gets stuck on this step. Before I added the ETSConfig line, it was the same error but with null instead of qt4. The whole traceback is below:

```
File "", line 971, in _find_and_load
File "", line 955, in _find_and_load_unlocked
File "", line 665, in _load_unlocked
File "c:\program files\python36\lib\site-packages\PyInstaller\loader\pyimod03_importers.py", line 631, in exec_module
exec(bytecode, module.__dict__)
File "site-packages\mayavi\mlab.py", line 27, in
File "", line 971, in _find_and_load
File "", line 955, in _find_and_load_unlocked
File "", line 665, in _load_unlocked
File "c:\program files\python36\lib\site-packages\PyInstaller\loader\pyimod03_importers.py", line 631, in exec_module
exec(bytecode, module.__dict__)
File "site-packages\mayavi\tools\camera.py", line 24, in
File "", line 971, in _find_and_load
File "", line 955, in _find_and_load_unlocked
File "", line 665, in _load_unlocked
File "c:\program files\python36\lib\site-packages\PyInstaller\loader\pyimod03_importers.py", line 631, in exec_module
exec(bytecode, module.__dict__)
File "site-packages\mayavi\tools\engine_manager.py", line 12, in
File "", line 971, in _find_and_load
File "", line 955, in _find_and_load_unlocked
File "", line 665, in _load_unlocked
File "c:\program files\python36\lib\site-packages\PyInstaller\loader\pyimod03_importers.py", line 631, in exec_module
exec(bytecode, module.__dict__)
File "site-packages\mayavi\preferences\api.py", line 4, in
File "", line 971, in _find_and_load
File "", line 955, in _find_and_load_unlocked
File "", line 665, in _load_unlocked
File "c:\program files\python36\lib\site-packages\PyInstaller\loader\pyimod03_importers.py", line 631, in exec_module
exec(bytecode, module.__dict__)
File "site-packages\mayavi\preferences\preference_manager.py", line 29, in
File "", line 971, in _find_and_load
File "", line 955, in _find_and_load_unlocked
File "", line 665, in _load_unlocked
File "c:\program files\python36\lib\site-packages\PyInstaller\loader\pyimod03_importers.py", line 631, in exec_module
exec(bytecode, module.__dict__)
File "site-packages\traitsui\api.py", line 36, in
File "", line 971, in _find_and_load
File "", line 955, in _find_and_load_unlocked
File "", line 665, in _load_unlocked
File "c:\program files\python36\lib\site-packages\PyInstaller\loader\pyimod03_importers.py", line 631, in exec_module
exec(bytecode, module.__dict__)
File "site-packages\traitsui\editors\__init__.py", line 23, in
File "", line 971, in _find_and_load
File "", line 955, in _find_and_load_unlocked
File "", line 665, in _load_unlocked
File "c:\program files\python36\lib\site-packages\PyInstaller\loader\pyimod03_importers.py", line 631, in exec_module
exec(bytecode, module.__dict__)
File "site-packages\traitsui\editors\api.py", line 24, in
File "", line 971, in _find_and_load
File "", line 955, in _find_and_load_unlocked
File "", line 665, in _load_unlocked
File "c:\program files\python36\lib\site-packages\PyInstaller\loader\pyimod03_importers.py", line 631, in exec_module
exec(bytecode, module.__dict__)
File "site-packages\traitsui\editors\code_editor.py", line 37, in
File "site-packages\traitsui\editors\code_editor.py", line 49, in ToolkitEditorFactory
File "site-packages\traits\traits.py", line 522, in __call__
File "site-packages\traits\traits.py", line 1236, in Color
File "site-packages\traitsui\toolkit_traits.py", line 8, in ColorTrait
File "site-packages\traitsui\toolkit.py", line 109, in toolkit
File "site-packages\pyface\base_toolkit.py", line 263, in find_toolkit
File "site-packages\pyface\base_toolkit.py", line 209, in import_toolkit
RuntimeError: No traitsui.toolkits plugin found for toolkit qt4
```