RAID 1(+0): breaking mirrors and rebuilding drives
How to, 3rd edition
http://h20000.www2.hp.com/bc/docs/su.../c00378986.pdf
Introduction
When using a configuration with RAID 1(+0) volumes attached to Smart Array controllers, it may be necessary to break a mirror and rebuild the drives. For example:
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When increased capacity is required, an administrator can replace half of the drives in a RAID 1 or RAID 1+0 array with larger capacity drives and rebuild the new drives. Afterwards, the administrator can replace the other half of the drives and allow them to rebuild in the other direction. Once all drives in the array are of larger capacity, the administrator can use the Array Configuration Utility (ACU) to increase the logical drive size or to configure additional volumes.
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For RAID 1+0 configurations, the first n/2 drives are always mirrored to the second n/2 drives.
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The drive-online LED will blink during the rebuild process. When it stops blinking and turns solid green, the rebuild is complete. On a 36-GB Ultra 320 drive, depending on the number of IOPS the server is submitting to the controller, it takes about 6 minutes to rebuild a mirrored array when the controller rebuild priority is set to high.
In your case, it sounds like drives 1&2 are being mirrored to 3&4. If so, it sounds like the basic steps would be: power down properly, remove 3&4 (and label them), power up, let the system know that you know drives 3&4 are missing, power down again, attach a new 3&4 set of drives, power back up and wait for the mirror to rebuild. Now follow the same procedure but with set 1&2.

Note: Recommend reading the "HP DL360 G5" documentation for the exact procedure.