I have one in Rf mount. I do use it occasionally and will use it in the coming weeks as part of the "Nikon Rf Month" project. It is a bit of an oddity - 3 element lens, slow and odd shape. Hood keeps popping off too (typical Nikkor lens hood). It is not a super hot performer at f4 and 5.6 - by no means bad, but it works best stopped down to f8/f11. Remarkably light though - particularly compared with the 105f2.8.
As for production, seems that they made about 700 in Rf mount and another 6-700 in F-mount. Difficult to find and the hood is scarce - as it kept falling off!
If you check our Flickr and use the tag "Nikkor 105mm f4.0" there are some shots off and with the lens.
I saw on Jonmanjiro's Flickr that he is using one for his trip through the Nordic countries and Estonia/Lithuania/Poland/ etc - this is ongoing as we speak so he will most likely have a lot of samples when he gets back home.