I used a 4 disc bag for a long time. The slots were:
1) All-purpose putt/approach disc
2) All-purpose midrange
3) Distance driver; best combo of distance and accuracy
4) Overstable driver, 'cause sometimes you just need one.
My picks would be:
1) Warlock
2) QMS
3) Valkyrie
4) Predator
He said mold, not disc, which I think is a big part of it. means you can carry 4 destroyers and 6 rocs in this scenario if those be your weapon. Some molds, like those, are proven to be very versatile as you will see looking in many pros bags (many of same mold).