Well, it would be difficult to determine on the Windows level whether
the single keystroke was maybe part of C-x M-f4 or C-h M-f4. So in
any case it is better to reuse emacs central event parser.
This is unnecesary too. Alt-F4 must work irrespectively of the prefix
keys typed so far: if you type C-x or C-h and then click on the Close
button on the top right of the frame, Emacs thinks you want to exit the
application. Alt-F4 must have the same effect as clicking that button
(when M-f4 is unbound on Emacs, hence the need for checking the binding
from the Windows event loop).