If you use a product like Adobe Photoshop, or Corel Painter, and you are not using a pen tablet, then you don't really understand what you are missing. A pen tablet allows you much more control than you can get with a mouse. It gives you much more efficiency in movement and the pressure sensitivity gives you the same kind of control that you would get from a pen, pencil, or paint brush.
The Intuos4 consists of a digitized pad that plugs in to the USB port on your computer. Your computer then treats the device as though it were a mouse. Through the use of a product like Photoshop or Painter that can take advantage of the rich technology that the pad enables, such that when you use the pen, you can apply more or less pressure which translates to various thicknesses and more or less affect on your image.
There was a time when the pen tablet was geared just for the commercial artist. They were expensive and the limited amount of software that was enabled to use the device was also geared toward the professional. Then came the Intuous3 which, while having a reasonable pricing structure, also seemed to work with more products, and now had traditional artists as well as photographers getting in on this technology.
Now enter the Intuos4 line of pen tablets. Not only has the price remained reasonable, there are now four sizes to choose from, and the quality of the product has improved dramatically. The total shape and surface have been completely re-engineered to give you more consistent strokes over longer periods of time.
I have worked with the Intuos3 (the 6 x 8 inch) for a number of years and it has become a mainstay in my workflow. Early on, that workflow was primarily photography where it has helped me with things like dodge, burn, sharpen, and blur effects as well as other techniques enormously. Over the course of the last couple of years, I have added Painter to my arsenal and would be hard pressed not having a Wacom tablet. The control is just phenomenal.