The ST4s is an awesome bike. My best friend picked one up this summer the day before I got the Tuono. I know that it is apples to oranges, but the Tuono is much more comfortable and the Aprilia motor feels a lot stronger according to the old butt dyno. The Aprilia engine is also smoother than the Ducati. On the positive side, the brakes are excellent. He has the ABS version and the the bike is confidence inspiring. It also handles like a dream at the limits. Having the Ohlins makes a huge difference in the qualityof the ride. It is a hoot to ride at sportbike speeds ( and feel more confident) on a sport tourer.
I have been on the Duc on several occassions and told my friend that the bike would be perfect for me if it had bar risers that were about 1 1/2" higher and 1" further back. You also need to maintain the batery on it. My friends bike has sat for the last 4 weeks, and now it won't start. He let the battery get flat and then recharged it on the tender and now it turns over fine but won't fire. The local Ducati dealer drove to his house and picked it up.
Good luck on your decision. There truly isn't a bad bike to be bought out there, just that these Italian bikes tend to need a little more care.