Your questions is a little vague / unclear.
I take it you are replaceing the hard drive in 4 of your office computers.
You do have the Windows XP keys for each of the computers.
You can use the keys you have , If you can obtain an install cd for XP of the same version as the keys are for. Different types are Retail, OEM, Branded (Dell,HP, Etc.)Corp/Vol. The key only works with the correct type of instlation media.
If these are Brand name computers , you may be able to get an installation cd from the manufacturer for a nominal fee.
If the keys have only been used the one time to install yor computers you should have no problem re-activating them with Microsoft at the time of the installation. Usually valid keys will activate without requiring even a phone call.