Curious on your title if you mean in place, as in to add raid, and not format existing disks. If so, this is not possible with any method I know, and if anyone tells you it is, make sure you have a backup. And if you have a backup, there is no reason to do it in place.
As stated, running raid z2 on 4 drives is not a great option, and raid 10 is a better setup. Same usable space, and generally a better setup with 4 drives.
Look at shucking drives possibly. I have recently built an array out of 3 10tb drives at $165 a piece for a raidZ (1 disk parity). I’m aware that it’s not safe, but I have critical data backed up on another array, so data loss is more of an annoyance than a concern.
Definitely overkill, so you are fine.
I’m assuming that you have a ton of media that you are storing. If not, please let us know so we have an understanding of what performance is needed. Generally speaking, more drives in a raidZ (Raid Z2 and Raid Z3 included) are faster with more drives, but that starts at 5 drives on a Z2, and 7 drives on a Z3 (compared to Raid 10). IF you are storing media, and not actively “working with data” on the drives, then generally it won’t matter.
I’ll also offer another option, that I WILL NOT RECOMMEND, but is something I have done in the past. You can build a degraded array using sparse files, intentionally removing parity until you add a drive in. I have done this before when doing a raidZ2 array, but did not have the budget for the last disk. I built it with a sparse file, and when I had the money for the last disk, I added it and resilvered. NEVER DO THIS WITHOUT A BACKUP! I can not stress that enough. Your data is NOT safe while you do that. In my case, it was as safe as a RaidZ (1 disk parity) but that’s not safe. The only reason to do that is budget, and only ever considered if you have a good backup in place. I knew I wanted a raidZ2, but couldn’t afford more than a RaidZ, and you can’t convert it after the fact without doing something like this.
TLDR on most of the above. Never ever do what I did, unless you have a backup. Hopefully I stressed this enough.