That's some great info, thank you. I did notice that it looks inside like it should open up more, like the aperture blades are being stopped at the f4 position artificially. At first I thought something was wrong with it, but I eventually realized it must be like that for a reason, because it stops down at every detent ( it works as it should) and you just solved this mystery for me, in that it's really an f2.8 lens that's stopped down from the factory.
I don't have any negs smaller than 35mm, but seems like a neat little lens to have, I guess.