Ideally Adobe recommend to uninstall the oder version before installing new one...however there are numerous people who hae both the version and the software works fine.
You can have LR4 and LR3 both at a same time on your computer
As Manish said, 2 versions can coexist peacefully, which might make some sense in case there are compatibility issues especially since LR 4 has a new RAW engine that may produce different looking results and you just need to get used to it. You can always uninstall the older version later...
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