Just played a 2.5k game with the new T4 rule and the difference was quite noticeable.
I had a group of 4 take on a horde of Plague Monks head on and destroyed them.
Well, the thing is - you would've crushed them with T3 almost as easily.
Horde of monks (with the banner) does 40 attacks, hits 20, rerols those for 30 hits, wounds 15, rerols 15 for 23 wounds, you fail every 6th for 3-4 wounds.
Against the T4 chickens, the math is like so:
Horde of monks (with the banner) does 40 attacks, hits 20, rerols those for 30 hits, wounds 10, rerols 20 for 17 wounds, you fail every 6th for 2-3 wounds.
You loose one less wound per phase on average.