MozillaBuild 4.0.1 is a small patch containing a couple bug fixes introduced in the MozillaBuild 4.0 Release. I advise upgrading it at your earliest convenience.
hg tab completions work again
shebang of 'Scripts\hg` now points to the correct python.exe
watchman has been upgraded to a much newer version (v2022.04.11.00)
The warning message for doing a “pave over install” of MozillaBuild now warns of the risk of instability for doing so.