The ST4S has four valve heads, the ST2 has two valve heads. More expensive cam belts, twice as many valve clearances to check and they also suffer from the flaking rocker syndrome ( dont get me started on that one again) which doesnt affect the ST2.
So give Nelly a bell (
www.cornerspeed.co.uk) and he will tell you what his fixed price servicing costs are for each bike to which you can add about £85.00 per rocker that needs replacing ( there are 16 of them!!) if you go for an ST4 or ST4S. On average its about three or four rockers a time.....