I had an S4 (with 916 engine) and was very happy with it. Plenty of pulling power and at the top end as well (though I know it had been detuned from the sportsbike 916 engines).
I've now changed to a Multistrada mainly because the Monster had a very small pillion seat and I wanted to do more 2-up rides with my wife (in comfort [it's a great place when you get there
] ).
Now having experienced both bikes, I have noticed the MS engine is better lower down in the rev. range whereas the S4 was a bit 'snatchy' below 4K revs. The S4 was great once the engine was spinning up, but then you weren't doing commuting speeds!! I'd agree with Clockwork Orange about the MS being a better commuter, but the S4/S4R will be closer in performance characteristics to your 996.