This is VERY good question, which we are surprised hasn’t come up before! We had to sit down and really contemplate the situation to make a ruling on this.
To recap, the Play All But 1 (New Rule) says “If you started with no cards in your hand and only drew 1, draw an extra card.” And, as we all know, when you play a card that increases the Draw amount, you get to draw the difference to increase your total cards drawn to the current New Rule in play.
The way Andy framed the question is “Is the extra card one draws like a ‘salary advance’ on your regular draw allotment , or is it more like a ‘bonus’ on top of your regular draw?” After some thought we felt that what the Play All But 1 card is doing is more like a temporary modification of the Basic Draw rule, and, as such, would make the extra card part of your total Draw allowance for your turn.
So, in the example presented in the question above, where (after having started with no cards, and Drawing 2) you have played Draw 4, you would draw only 2 additional cards (and continue to Play until you have only 1 card left in your hand).