The answer to your question depends entirely upon what silicon is in your server, and what the firmware revision is. Some Compaq servers (is this a Compaq or an HP/Compaq?) have on-motherboard SCSI controllers (or SCSI RAID controllers), while others have discrete add in boards.
How old is the server? If it’s really old (ie: more than 5 – 7 years) you may need to update the controller firmware. Are the drives in a RAID array or are they separate volumes? If they are separate drives AND the firmware revision of the silicon is current (check the manufacturer’s website – LSI or Adaptec for example), you should have no problem.
If this is a RAID array, you will need to grow the volume using the manufacturer’s software (or possibly you can do it from the controller BIOS…)
In additional as this is probably a very old server (HP/Compaq hasn’t shipped 4.3 Gig drives in many years) you may not be able to find newer larger hard drives in the correct physical size that your server needs them in. Your current drivers are probably large drives in a large white plastic drive cage. That form factor of hard drive was retired long before the HP/Compaq buyout. You can check the HP Parts website at http://www.hp.com/hpparts/ to see what size hard drives are available for your model of server. I went looking through the site and the largest drives which I could find for a server of that age are 36 Gig drives.
You may be able to get larger drives then that to work, but you’ll need to put them into the old drive cage, and they will need to have the same pin config as the old drives.