I am also using 192 dpi with a pair of 4k monitors, a 27-inch and a 24-inch, both at the native resolution of 2160p, so everything is crisp.
With the default font size set at 12 points, everything is very easy to read. Even 10-point or 9-point fonts are still comfortable, despite the fact that otherwise I need glasses for reading small printed books.
Under XFCE, almost everything is scaled fine on 4k and 192 dpi. The only exception is that some of the scrollbars remain rather narrow, but they are still usable.
With the default font size set at 12 points, everything is very easy to read. Even 10-point or 9-point fonts are still comfortable, despite the fact that otherwise I need glasses for reading small printed books.
Under XFCE, almost everything is scaled fine on 4k and 192 dpi. The only exception is that some of the scrollbars remain rather narrow, but they are still usable.