Answered December 24th, 2017

Highest completion percentage in 4,000-yard seasons in NFL history:

  1. Drew Brees, 71.9% (this season)
  2. Drew Brees, 71.2% (2011)
  3. Drew Brees, 70.6% (2009)
  4. Drew Brees, 70.0% (2016)

Highest completion percentage in a season with at least 4,000 passing yards
Interpreted as:
Highest completion percentage in a season with at least 4000 passing yards by a player