3. CRTC type 4 is the same as CRTC type 3. The registers also repeat as they do on the type 3.
== UM6845R and R31 ==
R31 is described in the UM6845R documentation as "Dummy Register".
It's use is described in the documentation for the Rockwell R6545 in combination with R18, R19 and R8 and the Status Register.
In the UM6845R it appears to have no effect. Reading and writing does nothing. Reading it returns 0x0ff.
R31 doesn't exist on types 0,2,3.
== UM6845R and R12/R13 ==