I've taken a shot of the sun using my Tamron 70-300 (at 300) on my Nikon D5000 at 1/4000 and F40 and had attached the ND400 filter.
The shot was taken mostly to test the filter so I ignored the dirty window through which I shot.
However I was surprised to find out that even with all the controls turned to minimum luminosity and having the filter installed the sun was too bright. Maybe the time of day (local time around 09:00 AM, today, November 29th) was inappropriate for taking such pictures?
Or maybe the filter is not enough?