The lace pattern does change the gauge. If you have the four inches measured out in pattern, though, you can see how many stitches it took to create those 4 inches. I think you'll be close enough. Maybe if you count and measure across the back along a ws row if it's purled?
I'm making a lacy sweater now which is supposed to be measured in pattern. I started with the sleeves and a needle size up since I generally have to do that anyway. I did measure after a while and counted the stitches on the needle and it was right and the sleeve measures what it's supposed to, so it worked.