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Thank you all for your advice, I just tested the sampling gate of my S-4
and believe it's good. Since I don't have a curve tracer, I improvised one
using a 7854, 7A26 and 7A22 driven by a triangle ramp from FG504 (-5V to
+5V, as the S-4 instruction manual says, 1.5kΩ for current limiting). I
took some fiddling to get accurate measurements, I uploaded everything to:
/g/TekScopes/files/S-4%20sampling%20gate%20diode%20test%20measurements

There are 4 sets of measurements:
01 - forward bias of both diode groups, 1mA/div vertical scale (1Ω current
sensing resistor), 2Hz triangle ramp frequency
02 - reverse bias of individual diodes, 10μA/div (1Ω current sensing
resistor), 2Hz triangle ramp frequency
03 - reverse bias of individual diodes, 1μA/div (10Ω current sensing
resistor), 2Hz triangle ramp frequency
04 - reverse bias of individual diodes, 1μA/div (100Ω current sensing
resistor), 0.02Hz triangle ramp frequency

Measurements 01 and 04 are valid (with minor offset errors), the rest may
be interesting for the curious. Measurement 02 showed that diodes C and D
are different from the rest, having a lower forward bias resistance.
Measurement 03 is with higher vertical sensitivity, and it showed notable
hysteresis in the reverse bias region of diodes C and D (what is shown is
the positive slope of the ramp). Reducing the frequency from 2Hz to 0.02Hz
eliminated the hysteresis (never would have thought that reverse recovery
could be so slow, but yet its <1μA we're looking at here) and increasing
the current sensing resistor to reduce hum/interference led to measurement
04.

The S-4 manual says "...vertical deflection factor of 0.001mA/div will
probably include some hum loops..." and "any one diode that shows any
discernible leakage is a cause to reject a whole Sampling Gate....". So if
I'm to trust my measurements, I'd say this one is alive and well? :)

I did not find any info on how to test the snap diode. Any suggestions?

Best Regards, Nenad