I have an Intuos4 (larger than the 4x6) and absolutely love it. It's the only tablet I've used so I can't compare/contrast it to others, but it has made photo editing so much easier.
One thing to consider when deciding which size to get is that the active surface of the tablet basically maps 1:1 with your screen. So if you're trying to draw a line (or, say, trace a contour) on your screen, you'll need a proportional amount of movement of your hand on the tablet. For a long line, a smaller tablet surface means you don't have to move your hand/wrist as much, but conversely trying to draw a short line means you need to accurately move your hand a tiny amount. For a large tablet the opposite holds true - you get better resolution trying to draw a "short" line but have to move your arm a fair bit to draw a "long" line. I'm not trying to push you one way or the other, just consider how you're likely to use it and what your comfort zone is.