I don't know why I had it in my mind that the 300mm would be f2.8. Now I wonder why they made it f4.
Perhaps because although its specified as a "300mm", given that it's an M/43 lens, it really equates to the 35mm equivalent of a 600mm lens! Nikon's own 600 / f4 currently sells for just a butt-hair shy of $10k and weighs in at over 11 lbs! mg: