John,
To add my 1p's worth, the ST4s now has the aluminium clutch plates so are considerably quieter than of old.
Never ground the ST2 out at Rockingham or in Italy on the passes.
ST2 seat was a bit painful (for a skinny git) after an hour but the 4s seat is slightly higher and seems more comfortable.
4s actually produces more power and torque than a 996 and haven't heard of any rocker problems with them.
With the TiN plated forks, adjustable Ohlins rear, a bit more ride-height and front compression it would take a real expert to find them wanting in the handling department, consumption? don't know, just fill it and ride it.
Brakes are excellent but can be improved further still with the 4 pad calipers.
Only downside? for me the p.xy little battery which I find needs to be kept on the optimate, then it starts first time.
There is a really good 4s forum with lots of info you will find helpful, not a link I'm afraid, but try
www.st4s.com We often make it over to Newlands or if you come to Popham you're welcome to try mine.