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.