Devastating Critical

You are a skilled combatant, capable of dealing out destructive critical hits.  You gain the following benefits:

• Increase your Strength or Dexterity by 1, to a maximum of 20.
• When you score a critical hit, add an additional die matching the weapon's damage die.  This additional die is to be doubled with the rest of the dice when applying critical damage.
• When you roll a natural 20 and score a critical hit, roll a second time.  if you get another natural 20, the critical damage multiplier becomes x4 instead of x2.