I don't understand your question, R3 and R4 are not on the same pin (R3 on pin4 is supply and R4 on pin5 is signal).
Consider that X3 is a Midi Thru port, is not necessary to play, it's for another Midi apparate that use the same played Midi Input.
Current of midi loop I've never measured, but as i mentioned in last post midi signal is elaborate to ZERO Volt; important is frequency of signal, but for technical detail I suggest you a google search.
If you make a search on the web, there are three similar project named:
-Popp Meyer MIDImal (german) 1997
-Elektor 1997
-MistralXG (the best-with USB port and not only) 2009 .......... many thanks to Steve that helped and stimolate me!!
P.S. I've tried CN17 and different 6NXXX and for my basic project are all good, the only difference it's speed of commutation - CN it's more slow, but we are speaking of microseconds so don't worry about this.
Mike.