I recently needed some 4" threaded pipe .... the plumbing supply house had no problem providing me with the pipe I asked for, cut to length, and threaded. Since pipe commonly comes in 20ft lengths, they ought to be able to make a piece for your mast - if you can transport it.
BTW, it may be a fine point, but I don't think a service mast need be made of 'listed' material. Conduit, yes ... but the code is silent as to the mast.
Failing a friendly supply house, any machine shop with an "engine lathe" ought to be able to cut the threads into anything up to 4" OD.