When Outlook pst files grow larger than 2 GB they run the risk corruption - especially prior to Outlook 2003.
It is safer to divide large pst files into two (or more) smaller pst files to eliminate the possibility of corruption and to speed up the pst repair process when needed.
If the data in those pst files is critical, back them up to a USB drive. Western Digital sells USB drives that include software which will automatically search for your data files and continuous back them up. Western Digital calls it “Automatic, continuous backup”.