I'd strongly second that suggestion. I can get iso3200 polaroid print film, but there are no high-speed alternatives for 4x5. Check the archives of the large format site or graflex.org and you'll see that every few weeks, someone is either asking for high-speed LF film for exactly the reasons above, or on a wild goose chase for the best way to push film to those speeds.
If, as a test, you could arrange to have someone make a run of good quality iso3200 b&w filme you could chop down to 4x5, I'd bet it would sell so quickly that it would justify making it a permanent inventory item.
I will look into the possibility and let you know. Thanks.