I have a RAID controller with 4 1TB disks and first I put them all in a RAID 5 array, partitioned 1TB for OS and 2TB for data.

Then I noticed that the raid array was never idle. It was constantly writing 24/7 and I figured it must be the swap file.

It feels like it would wear out all drives in the array faster and slow down other read/write operations.

Now I'm reinstalling the server and need some advice. Should I exclude 1 disk from the array (put it as RAID 0) for the OS and use the other 3 disks in RAID 5 for data?

In this configuration it feels like the swapping will only wear out one drive instead of all 4.

EDIT:
It is an HPE ProLiant DL60 with 8MB RAM and an onboard raid controller capable of RAID 0, RAID 1+0 and RAID 5.

The server will mainly be used as a file server and an SQL Server. It will also run some smaller services such as Subversion Server.

EDIT2:
Yes, I have googled and I didn't find any clear recommendations if an OS was better of on its own drive compared to being in the raid. RAID 5 is for performance not backup (and everyone knows that, right?!?). There is a backup to a single external USB drive twice a day.