I will see in real life how the unique S3 finder (in the Nikon RF world) compares to the M4...
It doesn't compare. The M4 viewfinder is 100ft over the S3 finder if you take it obviously (visibility of the framelines, contrast, resistance to flare, rangefinder patch general usability, automatic framelines display).
But, as you wrote yourself, you can realize that you won't take as good pictures with a camera which you don't either like or get related to.
I had an M4 and then re-sold it, because I much preferred the Nikon, for personal taste reasons (the smaller and lighter body, the way you load film, the all ball-bearings made shutter, the way you can see
through the (fuzzy, true) rangefinder patch for very, very accurate focusing, the incredible feeling of reliability which the camera provides). The Nikon S3 is the camera I now use almost all the time (I have an S2 and an SP too, but as very much as I like them this is the S3 which has become my 24/24 carry around camera - alongside with my Rollei 35 as for 135 film format).
Mine is the classic one made in the 1950s. I had the 2000 re-issue too but re-sold it. Its finder was not better than the classic one.
I have to wear glasses too, so the S3 built-in 35mm framelines are quite out of sight. I can live with that because I have the tiny Cosina-Voigtländer 28-35 external minifinder which is mandatory for using the W-Nikkor-C 28mm f/3.5 anyway, and which I leave on the camera all the time. With 35mm lenses it's either quickly removing my glasses to see the entire built-in 35mm frames for framing just before depressing the shutter, or more quietly using the 35mm frames of the minifinder. Both techniques work well for me. I don't find 28-35 external or secondary minifinders to be meh.