@Jim F - if you haven't already bought a 4" vise for your mill, I have a suggestion:
www.shars.com
It's $250 - a bit higher than some chinese offerings, but WELL worth the price. I recently bought their 6 incher and have been really happy with it. Works very smoothly, ground surfaces are superb, and even the un-machined inner surfaces are smooth - none of the rough, cobbly cast surfaces and sharp edges that I've seen on other vises. When the jaws close, you can feel the leadscrew stop abruptly. No mushiness (indicative of not-perfectly-parallel jaws).