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That really depends on how you want to setup your array, how many discs you have, and how paranoid you want to be. I, personally, prefer to err on the side of complete and total paranoia. However, my lowest level of paranoia requires six drives but four is doable.
If you want to you can include all four drives in your array and split the array volume into multiple partitions. One partition for the OS and one, or more, partitions for your other data.
You don't have to hold out one drive just for your OS. Including that drive will let you make use of more of your total drive space since you lose an entire drive in RAID 5. However, it's ultimately up to whichever configuration make you more comfortable and best fits your needs.
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I would definitely suggest that you NOT put your OS on the RAID 5 array for at least two reasons: OS speed/responsiveness and a desire separate the OS from your live/active data (failure/disaster control).
Although my suggested solution requires more than four physical disks as well (as does REadmin’s, above) (five minimum; six optimal), you might be able to get away with OS on a single disk and data on a RAID 5 array.
My suggested configuration? OS on a RAID 1 (mirrored) array, DATA on a RAID 5 array (giving you your 5 minimum disks) with one additional drive as a hot spare associated with the controller (not either array) which will automatically be pulled into a degraded array if (when???) a drive fails.
There are many other considerations as well, such as stripe and block size and (ideally) number of spindles (physical drives) drive type (SATA, SCSI, SAS, FC) and rotational speeds, but my impression is that you are not looking into these other issues at this time…