This got me thinking so I looked up Favre's stats. Funny thing, when you search for Favre on the NFL website under both active and historical players he can't be found. I had to go into a game log that I knew he played in to link to his personal page.
If you look at Favre in 4 year chunks this is what you get for QB rating:
91-94: 0, 85.3, 72.2, 90.7 ave (throwing out the zero): 82.73
95-98: 99.5, 95.8, 92.6, 87.8 ave: 93.9
99-02: 74.7, 78.0, 94.1, 85.6 ave: 83.1
03-06: 90.4, 92.4, 70.9, 72.7 ave: 81.6
07-10: 95.7, 81.0, 107.2, 69.9 ave: 88.4
Looking at totals, BF has had 2 of his best 4 years in the last 4 years. He has also had 3 of his worst 4 years in the last 6 years.