It is interesting that this 12 year old thread has not answered the question.
Disclosure - I don't have the Distagon 4/40 CF (it is on my list...), but I do have the Distagon 2.8/50 F that uses the same Series 93 system.
Here is the Hasselblad series 93 filter system
It consists of the base ring, which is the same on all lenses that use this, the series 93 filter in the middle, and the lock ring on top, which is marked for my lens. The lock ring for the 4/40 is slightly different, it does not have threads to attach any additional filters (and is differently marked).
The series 93 filter is thread-less and 93mm in diameter:
This is what it looks like assembled:
The thread on the lens that this adapter goes on is 86x1mm, and a filter with the same thread will fit - although the lens protection tabs on the 4/40 will interfere with a 86mm filter.
Also, keep in mind that 86mm filters came in
two thread pitches,
1mm and
0.75mm, so make sure you get the correct 86mm filter! I use standard 86mm (x1mm!) filters on my Distagon 2.8/50 F.
The front of
my adapter has a 95x1mm filter thread, where standard 95mm filters will fit (and those only came in one thread pitch). Again, the lock ring on the 4/40 adapter does not have threads.
For the Distagon 4/40, some people use a 86x1 to 95mm step-up ring and use 95mm filters. The step-up ring acts like the Hasselblad series 93 adapter and spaces the filter above the lens protection tabs.
100mm square filters are also an option using the 86mm thread.