So that the Replace With Clip -> From Source Monitor, Match Frame properly works, you have to properly position the playhead in the Source Monitor first.
I am aware of how this is supposed to work. Since All Footage is the same exactly thing as the Track 4 Only sequence, I should have been able to put the cursor at the same timecode and then replace and they line up exactly, but that isn't what was happening.
It is not just about how it is supposed to work. It is about how it does work.
What is more, you didn't have to copy your 'All Footage' sequence to the 'Track 4 Only' one, you could disable track 1-3 eyeballs right within 'All Footage', drop it into the Source Monitor and replace clips on your Track 2 within Master sequence via Replace With Clip -> From Source Monitor, Match Frame, since, as you noticed, disabling eyeballs inside nested sequence doesn't influence what you see in the multicam enabled track within Master sequence.