Oh the 4mm does make a huge difference.
If you are on a micro 4/3 whether it's a 4mm or 10mm, the difference is negligible. 17.5mm or 25mm is really splitting hairs. I have been using the 25mm Nokton, and i hve other widdies. There is only so much discrepancy to the naked eye after a while. At the end of the day, dun just go for the lens. A master craftsman can even make a pinhole useful