Since you're talking about a 4 foot span, I think you'll be much happier with multistart lead screws. When I order from McMaster-Carr, I usually get delivery in one day.
If you're interested, here's a link: http://www.mcmaster.com
Then search for "acme precision lead screw"
Many people are using 1/2 by 10 five start ACME lead screws. You can get the delrin nuts from CNCRouterParts.com or DumpsterCNC.com.