It's possible that something else is intercepting that keystroke prior to it ever filtering through to MS-Excel.

In looking at the NVDA Commands Quick Reference, under the EcoBraille section, it notes that F4 gets used for "Move to previous object."

When screen readers and other assistive software are involved there is a distinct keystroke interception hierarchy where if you're using something "above" the application layer that uses a given keystroke that's where it will stop rather than being passed on down to the application itself.

I just tested F4, with NVDA not in use and with NVDA active, and it did "repeat last action" in both situations.   I have a number of add-ons active, so it's not NVDA nor one of the add-ons I use.   There has got to be something active that expects the F4 key for some other purpose and that intercepts it, preventing it from making it to MS-Excel.
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Brian - Windows 10 Pro, 64-Bit, Version 1809, Build 17763  

Growth for the sake of growth is the ideology of the cancer cell.

           ~ Edward Abbey