I believe that changing the RAID array from RAID0 to RAID10 requires full reconstruction and the contents of the drives will be lost in the process.
In my opinion, once you have 4 drives, RAID5 makes much more sense. You are just as safe from a single drive failure and you can store more data in a RAID5 array than you can in a RAID10 array (which uses only simple striping and mirroring) using the same set of drives. I cannot remember whether the Intel Rapid Storage implementation on the DP board supported RAID5, however.
Last but not least, I caution you to remember that we are talking about a board and silicon that is 8+ years old. This is beyond the standard 7-year MTBF that is used with such equipment and age-related failures are bound to eventually happen. I would certainly recommend that you take backups regularly...
Hope this helps,
A few things I'll add to what Scott said:
RAID 1 to RAID 10 is not a supported migration for ICH9R: http://www.intel.com/content/www/us/en/support/boards-and-kits/000005997.html
If you aren't already, you should be using RST driver 9.6 or higher (up to about 10.8 or maybe 11.2) for 4k sector support (512e / Advanced Format): http://www.intel.com/content/www/us/en/support/boards-and-kits/000005646.html
Even if you are using an RST driver that is compatible with 4k sectors, the RAID Option ROM in the BIOS tops out at 188.8.131.523, so you may run into issues there (when creating the array, if using the Option ROM).
The reason I say *maybe* RST 11.2, is that ICH9R support disappears from the iaahci.inf in that version. The RAID device ID (PCI\VEN_8086&DEV_2822) in iastor.inf stays the same, however, and will install regardless. RST 10.8 has a listing for the ICH9R in the iaahci.inf, so support isn't an issue there.
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Thanks for the help on this; Please dermeister let me know if it helps you, also if there is something else I could do for you.