The primary benefit for RP4 is that it is co-located in a subrack with the Extension Modules (EM's) that it controls. In AXE10 RP's control EM's and EM's control devices.
The RP4 is only found in BYB501 / AXE810 generation of AXE10 and does away with parallel buses, instead having a serial bus cable as seen above RPB S/O and RPB S/I. (Serial out and Serial in)
These buses are actually cables that are cabled directly back to the Central Processor (APZ212*33 in the above case)
CP controls RP4 which controls the EM's which contain (and control) devices. A device can be any entity that provides a service such as a line circuit (LIC) on a line board (LIB) for instance. An example of a LIB in AXE10 is ROF 137 4442/1 LIC12.