So basically if you only ran 1 and 4 you would have the whole range ?

What about doing just a two speed pulley using a 6 rib Poly vee belt drive and that way you could get the smaller pulley on the intermediate shaft a tad smaller without loosing wrap round as Poly vees' don't suffer from this like V belts and you could probably get to less than 100 rpm.

Mind you if you are on CNC then it's a different ball game to the manual owner who needs low revs for large cutters and boring.