No clue what the rules of your league are, but if you are in a "flex" league, where you can start RBs or WRs at 2-3 "flex" positions, you're making a HUGE mistake going with 4 WRs. Sure, at the end of the year WRs will put up similar numbers to RBs, but week in week out they're nowhere near as consistant as RBs...and if you're playing head to head consistancy is the key. A Starting RB is going to see 15+ touches a game...your WR might see 1 catch some weeks. A 4 WR offense, no matter how good your WRs are is the recipe for an up and down season. One week you might put up more points than anybody else all season and the get smacked by the worst team in the league because your 4 WR combined for 7 catches, 120 yards and 1 TD.
If given the option ALWAYS go heavy on RBs...well, at least that's the way I see it =)