+1 to what Tony said.
They overhauled the development process engine for LR4. The new process 2012 is better than the older one in LR3. In addition in LR4 they introduced the Adjustment Brush, which in my view is worth the cost of the software on its own. It allows you to select just part of the image and change the White balance, exposure, shadows, highlight, saturation etc while leaving the rest of the image untouched. - You can of course have several of these on one image so you can reduce the exposure on a bright sky, while also increasing the exposure of a darker area, without making any changes to the rest of the image.
LR5 improved the spot removal tool (it still isn't as good as Photoshop) and added a new Straighten tool, which works to reduce/eliminate keystoning. Used it on several recent images of buildings and it works very well.
In short I would certainly recommend upgrading.