I once owned an R3ii but sold it shortly after I bought it because its power button was too easy to touch accidentally. As for the sound, the M1 Plus is better, with a better soundstage and better sound details.
I haven't tried R4 yet, so I can't compare the difference with you, but I have R6iii. R6iii provides more sound details and better bass, but it is not easy to carry. In comparison, M1 Plus better meets my needs. , and the price of R6iii is doubled. That’s all I can say at the moment. More detailed differences can only be provided after I experience the M1 Plus in depth. By the way, my earphone is Aful Performer 8.