ok, you can take the sd4500 off the list, it is only 36mm at its widest, so it would not be a good choice for landscape shots.
that narrows it down to the s95 and sd4000. the s95 is the clear choice if you do shots in low light, and while it offers a wealth of manual controls, you can always just shoot it in an automatic mode, or even better shoot in P mode and control the ISO yourself.
the sd4000 is a nice compact as well. it won't handle low light situation as well as its bretheren, but it does better than most compacts. and will handle normal lighting situations very well, and saves you some money over the s95.