If you can match up 4 layers from the valances, and if the fabric hasn't faded too much, go for it! It is supposed to be easier to do any of the layering techniques (4PP, OBW, S&W) from fabric that hasn't been washed, and also from fabric that hasn't been cut. However some people have said on the board that they used fabric that had been washed, and 4PP doesn't require as many repeats, so you may have come up with a great idea, and it sounds like you will be getting new window coverings as well, so it's win-win.