Just so I can think this through, you'd end up with the 4th and 6th pick, plus an extra 4th rounder, and you'd lose your 2nd and 3rd round picks? So in a 12-team league you'd get the 6th and 43rd pick in exchange for the 21st and 28th pick.
I just completed a 12-team draft, and here are the selections made around those picks:
The difference between the 6th and 21st pick is going to give you more value than what you lose between the 28th and 43rd pick. 3rd and 4th rounds also start hitting the no-man's land where tough choices get made and dice get rolled. At pick 6 you are (hopefully) getting a sure thing.viewtopic.php?f=18&t=492310