Don´t know if it will still apply to CVI 4.0 (and NT) but that´s how i do it.
In the receiving Application:
iMessage = RegisterWindowMessage(MY_EVENT_NAME);
RegisterWinMsgCallback (clb_MyEvent, MY_EVENT_NAME, 0, 0, &iCallbackID, 0);
And in the Calling Application:
iMessage = RegisterWindowMessage(MY_EVENT_NAME);
PostMessage(HWND_BROADCAST,iMessage,0,0);
Hope it helps.