But what's the value of C4 ...?

I don't have a manual for this old incubator, but later models (C-100 et al) quote various values in this position.

However, I did notice that surrounding caps seem to have similar (or the same) values.

So my suggestion would be to have a look around those nearby components.

Failing that, take a look at the circuit (and what C4 is supposed to do), and make a best guess at its value. In other words:- an engineer's approach! smile

But lastly, are we sure that C4 is faulty? think

If you don't inspect ... don't expect.