Screws have problems with whipping at the speeds you'll want when they're that long. 4' of travel is about the max unless you get some really large diameter screws. I'd highly recommend looking at rack and pinion. Check out cncrouterparts.com for example. You can make 2 motors drive at the same speed. That's how I run my router (with screws) and how many other people here have their machines set up. It's called 'slaving' motors together in Mach3. You also get the advantage of twice the force to push around the heaviest component on the machine. You can also set up individual home switches so you can square the gantry every time you home. It's a pretty cool way to go.
CNC mill build thread: http://www.cnczone.com/forums/vertical_mill_lathe_project_log/110305-gantry_mill.html