In line with the suggestions from virtually everyone, I ordered two of each variety... (magnum, cascade, centennial, and willamette)
Do I plant them in pairs, or do I have to plant them individually and keep them separate?
Plant them a few (say 3) feet apart for the same variety. Plant different varieties further apart, so the vines don't get tangled up in each other. You don't want to plant all different varieties only to have them all get mixed up.