Yes, it features professional dust and moisture resistance. However, Canon recommends using a front-mounted protect filter to complete the weather seal at the lens front.
Yes, it works perfectly on EOS R, R5, R6, and other R-series bodies when used with the Canon EF-EOS R Mount Adapter.
While the f/4 aperture is constant, it is better suited for daylight or tripod-based long exposures. For handheld extreme low light, an f/2.8 lens may be preferred.