IMO, article 4 of amendment 25 has a much higher chance than Mueller

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2. 25.4 ain't happening unless Donnie is in a coma

It just isn't a viable means to get rid of a bad president. You need cooperation from a cabinet where loyalty is usually a factor, and in Donnie's case it seems like the only factor.

Appearing before a grand jury, on the other hand, seems quite likely to be required, albeit probably on tape and almost surely with Donnie exclusively invoking the 5th or "not remembering".