I meter my MGIV at ISO 6 and have acceptable results without any filtration. on a pinhole with an fstop around f/345, focal length of 100mm on a 'sunny 16' exposure its normally around 1minute 30seconds.
When putting a filter on it, I dont recall the exact numbers but I believe 0 - 3 all have a paper ISO around 200, and 4-5 are 100. Without a filter as mentioned its 500. Using these measures you can adjust your exposure accordingly, a #5 contrast filter puts the paper at 100 'iso' and a #2 puts it at 200 'iso' ..... at least in my experience doubling and halfing exposure has seemed to work just as it does with film ISO..