Depends on the phone you have.
Check the rooting sub-forum for your phone model. If the manufacturer has issued a ROM for v4.x, I'm sure there will be information on how to get it. Alternatively, if it's a popular model, a 3rd party may have produced one.
Note that rooting may invalidate your warranty .. and that most of the cool stuff no longer comes via the Android updates, it's nearly all in the apps so you'll have it anyway.
