All depends on your league all leagues have different scoring. that just happens to be standard.
My guess on y that is that is to even things out a QB is gonna throw 3000-4000 yards a season and 20-40 TDs, as well as throw the ball 30+ times a game.
A RB on the other hand only carries the ball 20-25 times a game and 1000 yards an 10+ TDs is considered a great season.
If you look at some of the greatest rushing seasons of all-time, LT's, Dickerson's, Faulk's, Chris Johnson's, etc. They scored 20+ TDs and of that group only eclipsed2000 yards twice if a QB throws for 20 TDs and 2000 yards thats a Mediocre season when its a great when for a RB.
I dont like the 4 pts per TD for a QB personally in all my leagues(1 of which im commissioner in) the QBs get 6 pts a TD and 20 or 25 yards per point.
Im just giving you the reasoning behind 4 pts a TD being the standard
my main league has rushing and passing td's at 6 points. RB's go earlier than QB's due to the fact there's only a handful of elite rb's. And I believe 10 QB's threw for over 4,000 yards last year. So QB is deep this year at least. But if i'm in the last few spots of a 10 or 12 team league I'll grab an elite WR and QB first 2 picks then get a Foster/Bradshaw/Bush ect who are great in ppr leagues to fill my RB. It's different every year though.
Yeah last year it was my strategy to pick 3 HIGH END Qb's, because in our league they basically had the same value, if not more, than RB's.
In our 12 team HF Sharks board league I went AJ, Moss, D-Jax my first 3 picks. The 12th pick took Rodgers & Brees back to back. He flipped me Rodgers for AJ. So now I have Moss/D-Jax/Rodgers. Pretty happy especially since I have Finley too.