Go for the Intuos. I haven't played with the Grapphire 4, but I hated the Grapphire 3. The overlay was too soft, and gave way when pressing hard.
The Intuos is far more solid.
And with the latest Intuos 3, the pressure sensitivity is higher, at 1024 levels of pressure. And the resolution is higher. If you do anything other than photo retouching or graphics logos, you will appreciate the extra levels of pressure.
If you are looking more for a budget, look for an Intuos 2 on eBay, but stay away from the original Intuos if you use a CRT monitor, they get some nasty interference from them, making the cursor shake like crazy if you get too close to the monitor.
Basically comes down to your budget. If you can, I'd say 6x8 tablet should be the smallest you should get. 6x8 is a great size to start with, the 4x5's are just too small, even if you are working with a tiny 15" monitor.
Of course, I been using Cintiq's for years now, so I really can't go back to a standard tablet, but uh...yeah, it's a bit more spendy....
-Ed