The brightness and contrast in the finder in older Leicas really depends on its condition. Mind that even the youngest M4 is now 40 years old... M3 and M2 even older.
I have a M2 from 1964... after cleaning the surfaces of the finder it's bright, clear and contrasty enough (better then the last M9 I looked through 😉 ).
The patch never flared out on me and never disappears, it only looses contrast slightly depending if your eye is centered or not.
The finders of the older Leicas are slightly tinted blue (which makes them appear slightly darker) with the framelines and patches tinted yellow. It helps with contrast as both are complementary colors.
But I also have never looked through a Bessa, I can't compare the finders.