Sheldon Brown writes this:

"Most wheels are built cross 3. Higher cross numbers cause the spokes to leave the hub flange more nearly at a tangent. This makes them better able to withstand the twisting forces of hard pedaling in low gears, and also braking forces in the case of hub brakes. "

http://www.sheldonbrown.com/wheelbuild.html

"[H]ard pedaling in low gears" sounds like climbing a hill with a loaded touring bike -- I'd go for the 4-cross pattern.