I have my 4S on ST4 gearing - 15 / 43 on Renthal sprockets.
This gives a 100mph cruise at 6000 rpm, top end still well over 140mph and to say it's fun out of the bends is an understatement
Combining this with the oil rig strength Tsubaki chain that Nelly supplies and a Scottoiler and I am getting over 2000 miles between chain adjustments.
Personally, I'd go 15/43 - less effect on your chain from the smaller radius of the front sprocket, slightly more clearance between the top run of the chain over the top chain runner on the swingarm and that big rear sprocket looks so much better!
Andy