Assuming the 4 disks are the same make and model, or at least similar, you want to set up a raid. You can do this in the server installer. Create a partition on each disk designated as a raid component, then create a raid array out of the components. You can then install the whole system onto the raid array, but it is a better idea to use the raid array as an LVM physical volume, and use LVM to divvy up the array into different logical volumes, and leave some space for later use. If you care about handling the possibility of one of the 4 disks failing, then you should use either raid5, or raid10. Raid5 will give more usable space, and better sequential write throughput. Raid10 will give better random IO throughput and may even handle a second disk failing, depending on which one it is.

See wiki.ubuntu.com/Lvm for more information on lvm.