I have the Wacom Graphire3 4x5 on my laptop now the Intuos3 (6x8) on my main workstation. Although both are excellent, the difference in size, tracking resolution, accuracy, and pressure sensitivity is rather pronounced in moving from one to the other.
There are other benefits though that are not immediately apparent when owning Wacom tablets: discounts on major software titles such as Adobe Photoshop and Corel Painter. Depending on which tablet you get, you get certain free software titles and versions. With Intuous3 it was Photoshop Elements 3 and Corel Painter Classic 2 I think.
Elements 3 is an amazingly capable program, very close to CS line in terms of features (although Elements 4 is closer yet apparently and out now). Any of the Painter programs are very capable and work well with the tablets. Because of those “Wacom privileges” I was eligible for substantial discounts on Photoshop CS and Painter IX. Some of those may be time limited offers though, so they might not always be available.
The Intuos nibs last long, and the wireless mouse that comes with either is a cool addition but not always comfortable or as responsive as an optical mouse it seems. The Intuos3 mouse has extra buttons that I wound up deactivating because they were not ergonomically situated and could activate submenus and functions mistakingly if you weren’t careful.
The pens are where some of the biggest differences exist between the Graphire3 and the Intuos3. The Graphire has a slimline pen, whereas the Intuos3 three has a thicker pen cushioned where you hold it. They now sell the thinner pen for the Intuos3 at the Wacom website, along with a new “6D brush” that transmits extra pivot information through the tablet to a graphics program like Painter. These are not necessary, but if you invest in an Intuos you might consider those new options.
The larger Intuos tablets (6x8 and higher) do seem to allow for cleaner and more natural pen strokes I find, as they’re not forcing you to compress your motion in a smaller space, and they seem to track faster at a higher resolution, making for overall smoother lines.
I just checked the Wacom.com website and apparently the Graphire4 is out, even a wireless version (but with older software apparently).
The Graphire series is still a great purchase considering the dollar amount, and the 4x5 is perfect for portability. Either one should make you happy 
RobertT