Having just done a full restoration on a C4 and lived and breathed the schematic for the past month it is much more than a no-slouch, it has a superb phono section and is a superb preamp, hands down.
The phono stage is extraordinary. It has an excellent RIAA EQ circuit. It is quite accurate, it does not implement the IEC time constant, and instead of having a Neumann pole at 50kHz, it has a pole at 72kHz. It also has a superb and incredibly rare input noise reduction circuit at the input to the balanced FETs which reduces Johnston noise by around 3db. Instead of having the typical switched resistor banks at and around 47kOhm input resistance, it has a 1MOhm input resistor which is fed by an inverse RIAA (to reimplement RIAA encoding on the decoded signal) which is fed negatively at a gain of around 20 such that the input resistance 'looks' like 47kOhm. Absolutely brilliant.
The RIAA EQ is excellent as is, but if you calculate the transfer function on the feedback circuit, it can be improved. With a little maths, I was able to set the poles at 3180.0001us, 317.999994us, 75.000001us and the residual pole I shifted up to over 75kHz thereby reducing it's effect somewhat.
I recapped throughout with WIMA's and Nichicon FG's and MUSEs . There had been a lot of drift in the caps over 40 years. I also found that most resistors had also drifted dramatically over time, so I replaced all caps, all resistors and all diodes. I would suggest anyone rebuilding a C4 do the same. I was expecting to have to replace some semiconductors, but they all tested fine.
I also dismantled all switches and pot and thoroughly cleaned and deoxidized their contacts back to gleaming silver and now they are all silent and silky smooth.
The Yamaha C4 sounds absolutely superb. It is one of the most accurate and natural amps I've ever heard and upon running sweep voltages and RIAA encoded sweeps through the phono sections its not hard to see why, it is absolutely flat from 1Hz to 100kHz.
If anyone wants to upgrade their RIAA EQ circuit let me know and I'll let you know what components to replace.