The Second part, we can only remain active IF we pass a willpower roll at 8 at a -4 dice penalty. By Pain Reforged and By Agony Empowered both stop the effects of Bashing and Lethal damage in the final health level. But where By Pain Reforged is automatic, AND reduces wounds by -1, Agony by comparison, requires you to pass a DC 8 Will roll at a -4 penalty. If you fail, the charm essentially fails as you are knocked out in a fight close to something that put you down in the first place and it allows you to spend 1 essence to get a max of +4 dice for one turn (Since Pain reforged reduces -5 penalty to -4).
So the "Double your health level" is very much conditional on a dice throw. Literally. The only advantage of Agony over pain reforged is that it works on Lethal Dice and gives us the "chance" of being able to continue fighting. If it did not have the Roll DC 8 Will then yeah, 4 Dots would be ok but the charm, well, is meh. Especially since it still compares very badly to 3 Dot charms from other splats.