If there are no loading options on the front or back of the RM-4 leave it alone! I'm sure that the late Rodger Modjeski knew what he was doing when he designed/built it!
Music Reference RM-4 Input-Loading and Gain-Select plugs
I have been using Music Reference RM-4 pre-preamp with MCC cartridge Denon DL103R for quite some time. This cartridge has an output voltage of 0.25mV and output impedance of 14 ohms. I am aware that the Music Reference RM-4 has some switchable Input-Loading and Gain-Select plugs. Unfortunately, I have purchased RM-4 with no user manual. And I was not able to locate one on Internet. I wonder what those Input-Loading and Gain-Select plugs are and how they could be used to match RM-4 to Denon DL103R cartridge "better". I would also appreciate any help in getting a copy of the user manual (regardless if it is PDF of just the mobile phone photos of the manual pages).