Using 64-bit WinPython 3.5.3.0Qt5 on Win8.1, I did the following:

1. Download from http://www.lfd.uci.edu/~gohlke/pythonlibs/#pyqt4
2. Copy file to D:\toto\PyQt4‑4.11.4‑cp35‑none‑win_amd64.whl
3. Run "WinPython Command Prompt.exe" as administrator
4. In the cmd.exe window, type >wppm -u PyQt5
5. In the cmd.exe window, type >wppm -i D:\toto\PyQt4‑4.11.4‑cp35‑none‑win_amd64.whl
6. Run WinPython Control Panel with elevated privileges

The result is that WinPython Control Panel process terminates. I worked around the problem by deleting the folder "%WINPYDIR%\Lib\site-packages\PyQt5".

Perhaps the problem occurs because PyQt5 is in the path before PyQt4, and uninstalling PyQt5 leaves a DLL. How about changing env.bat to use different paths for the PyQt5 and PyQt4 cases?

By the way, I am using PyQt4 because I continue to have this problem: matplotlib/matplotlib#7688
I would have re-opened this issue: #449
But the WinPython project does not allow users to do this.