I find the performance of the SD4000 at high ISO settings to be quite good. Check out the link below. It would be incorrect to compare the S95 to the SD4000, as the S95 has a much larger imager, though they both have F 2.0 lenses.
I opted from the SD4000 as it was more budget compatible than the S95. All in all I have been impressed with the SD4000's performance.
This photo was taken on the run at ISO 1600, and I am impressed with the result: