You do not need to keep older versions of LR after installing LR 4. After you've installed it, if you open your old catalog, LR4 will make a copy of the catalog, converted to LR4 format. The images will be imported, with their current process version. LR4 is capable of reading and processing the process version 2010 (PV2010) as well as both the older version and the newer version, PV2012.
I've not converted most of my older images, but those that I have, are much superior to PV2010 and with much less time and work.
Alan