not much diff between spec2 and 3, other then the vtec opening, the meter and the obvious tail difference (which can put fixed on spec2)
About s4 it's a good bike, rode 1 for 3 years... never gave any major problem during my ownership. It's quite an overrated bike tho, I definitely won't mind riding one again but if I have to choose to ride an inline 4 400cc bike again, I would go with gsr4...