As I understand, ND4 filter should transmit about 25% of the incident light. I have measured transmittance spectrum of my Fujimi ND4 58mm filter, and got the following result:
So, transmittance is about 40%-45% in the visible range of wavelengths. That's quite far from the expected 25%. Is my filter not really ND4 (although I bought it as such, and it has the marking on its rim)? Or is my expectation wrong, and this filter is normal?
EDIT: I've done some more tests:
Luxmeter: covered with the filter it shows 39% of its no-filter readout.
Canon EOS 1100D manual mode metering: without filter I get the meter at the center with exposure of 0"5, and with filter it goes 4 steps lower, so that to compensate I have to click the wheel 4 times, getting exposure "5" (i.e. 1/5 s). The ratio (1/5)/0.5 is thus also 40%.
Canon EOS 1100D raw photo: raw histograms of the photos with the same exposure have corresponding peaks at values having the ratios 38% to 41%, depending on color channel (corroborating the tint visible in the transmittance spectrum).