The switch from Bamboo Fun to Intuos4 is heavenly. I can tell the difference right from the start Let me compare a bit.

Bamboo Fun - Small
It is cheap, and after intuos4's launch, it has became much cheaper. The interface is smooth. Initially when i used the bamboo, the surface is kinda rough, but after a few months of drawing, the surface has become smooth. In a way, the pen tip will experience minimum friction now. After one year of usage, i finally change its tip into a new one included inside the package, so it's like, the pen tip is very very durable.

Intuos4 - Small
The surface is much more paper like. I think it's a little bit better than the surface for intuos3. The stylus's design and weight is great, but might be tiring after sometimes of usuage. This part, Bamboo Fun's light weight pen might win over. But the best thing is that the Intuos4 has the best sensitivity available! 2048 levels! My curves and lines are much more realistic as compared to Bamboo Fun. It really can capture the slightest touch of my hand. I'm not really sure about Intuos3 stylus, but intuos4 is great to me. Imagine that my strokes can now be captured with 4 times the sensitivity, cool~

Conclusion
Looking at their pricing and products i believe Intuos4 is only a slight improvement to Intuos3. You'll need to consider the fact that cintiq also uses intuos3 techonology (they uses the same stylus i suppose). So my guess is that when it's time for intuos3 to be obsolete, intuos5 with a totally new technology and improvement will be out. Estimated time : 5years

Some words
Currently there aren't any good tablet to be comparable to intuos3 already, except for their pricing. Now Wacom came out with intuos4, i guess no one can cover this big gap in this tablet marathon anymore? I live Wacom's table Intuos4. Love it