>When we restore database from backup file in one hard disk system. That backup file size is 4 GB. It take more than 3 hours. Which kind option can we us to make restore faster?
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>I also find the restore is much slower in the server with hard disk array system or raid 5 system than one hard disk system. Is it normal?

Oh yes, that's normal. Raid5 has to spread the data over the (three) disks, has to do some calculations etc. Obviously this doesn't account for restores only, but for any normal writing apps.
Have the disks duplicated instead. Saves a disk as well... (NT (like Novell) can do this from nature, though the hardware needed is harder to find nowadays).
Both Novell and NT gain from elevator seeking; at retrieving the data it's faster than Raid5 too.