That’s fine for getting 4″ distance accurate but not necessarily for achieving the correct tension, as this might end up at 3 1/2″ or 4 1/2″.
My suggestion , (and it’s only a suggestion!) is set the 4″ as described above and then take the genoa down and rehoist it UPSIDE DOWN!! Stand around for a few moment whilst all the smart bottoms (there might be children reading this) come up to you, wetting themselves with the pleasure of being able to tell you what a plonker you are, only to have the wind taken out of their sails (pun definately intended) when you explain that quite obviously you are adjusting the tension of the luff using the lanyard at the top of the sail, thus retaining an accurate 4″ at the tack. Tape the lanyard up, as once set you shouldn’t need to do it again.
But don’t forget to take the sail down again and rehoist or you really will look silly as you unfurl on the start line!!
Ed: Yes, I have also done it in exactly the way Ian suggests for many years….. and yes, much laughter prevails!! My father though tells me that hoisting your jib upside down is an internationally recognised distress signal…. so if you sail at a sea based Club, take care you don’t launch the Lifeboat !!… datchetman