4 computers with 4 coworker, or 4 computer and you are the only one who work
on this computer?
I think you don´t have exchange, or do you?
Some general information:
4 computers with 1 user who change only the computer:
You could work with only one PST file on all 4 computers but you have to
close your Outlook on the computer if you move to the next computer. Have a
look on this sie for the briefcase solution: http://www.slipstick.com/outlook/sync.htm#briefcase
Your "Briefcase" could be a central folder on a shared Harddisk folder but a
network PST is not supported by Microsoft: