I just tried setting my OM-4T to ISO 3200 and setting the lens to f/2.8.
With the lens covered, I get a reading of about 1/4 s. When I aim the
camera at a mid-tone in my bedroom (which is lit by a single compact
fluorescent lamp), the reading jumps to about 1/60 s. This means the
"dark exposure" is about 60f the correct exposure, an insignificant
amount. In dimmer light (1/30 s, about the slowest speed you can safely
hand-hold), it would be about 12%, still an insignificant 1/6-stop.
I assume the OM-4T uses a photo diode for its TTL (ie, off the focusing
screen) readings. Any photo diode will have a very weak output, even in
"zero" light. * This is (probably) why you get a reading even when the
lens is capped.
The question this raises is just how the camera can provide accurate
(ingnoring reciprocity failure) four-MINUTE exposures in OTF mode. Does
the OTF mode use a much-higher-quality photo diode? Or is there
something else about the design of the camera that negates the problem
of the dark current?
* The photo diode has a finite resistance, and is not at absolute zero.
It therefore produces a tiny current.
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