..."Did you manually copy the *.dlls?"
Yes I did. In fact, I just re-downloaded the current Input Overlay V4 zip file again and copied the .dll's manually to the 64 bit folder: "C:\Program Files (x86)\obs-studio\obs-plugins\64bit" and it still won't load into OBS.
Strange, this is very puzzling. By the way, I'm running Win 8.1, not Win 7. Oddly enought, I do see (in the 64 bit OBS) the Source entries for the "Input Overlay" and "Input History" that I added while running the 32 bit version of OBS, but when you double click them to reveal their settings, the dialog boxes that pop up are just blank. Also, when you click the + button to add a source, neither Input Overlay or Input History appears on the list of sources.
At first I suspected that it might be an issue with windows 8.1 being touchy about 64 bit .dll code being executed in a folder location pre-designated for 32 bit applications, as Win 8.1 has two different programs folder locations for 32 & 64 bit software. However, both OBS versions (32 & 64bit) were installed by default in the 32bit (x86) program files location, "C:\Program Files (x86)\obs-studio", and the 64bit version of OBS seems to run just fine from that location. Are both 32&64 bit versions of OBS supposed to be installed in the "C:\Program Files (x86)\obs-studio" folder?