The S90 has a larger CCD sensor than found in most compact cameras. Combined with its fast f2.0 lens at wide angle, it does remarkably well in low light for a small camera.
The SD4000 has a smaller, backside-illuminated CMOS sensor. It also has a f2.0 lens and provides respectable low light shots. The CMOS sensor also allows faster continuous shooting speeds than available on the the S90.
Overall image quality
The S90 has a edge on overall image quality and sharpness. Like the S90, SD4000 images will benefit from using minus Exposure Compensation when photographing in bright light with high contrast. Neither camera has the dynamic range of a DSLR.
Images from the S90 are generally sharper than the SD4000, particularly at the longer end of the zoom. If you want to increase in-camera sharpening you’ll have to use P or another mode to access the Custom My Colors menu since it is not accessible in full auto mode. When using single area focus, images generally come out sharper than if you use a multi-point focus mode.
The S90 does a better job at controlling noise at higher ISO numbers. For the SD4000, minimal noise is present even at the lowest ISO number (125). Neither camera does well at higher ISO numbers than cameras with large sensors such as DSLRs and Micro-Four Thirds.
However, except when shooting at ISO 1600 and above with the SD4000, noise isn’t generally a major visual problem after an image is reduced in size and sharpened.
The video quality of the SD4000 is much better than the s90. It has HD video, which the s90 does not. You can also zoom with the optical zoom as you shoot with the 4000. If you want to zoom while taking video with the s90, you’ll have to move yourself closer or further away from the subject. You can also use the digital zoom while taking movies, but image quality may be degraded.