Purchasing Organization

Hi all,

We are currently in Blue Print stage, i want to know my client have 4 plants situated in single city and we have assigned them to single company code.

Now i wnted to know while defining the purchasing organization should we define one purchasing org for each plant or single purchasing org. for all plants belong to company code.

What should be the assignemnts in both cases? Which one of them is preffered and why i.e. having 1 pur.org for all plant or else?