First, L2ARC vDevs can not be Mirrored or RAID-Zx’ed. L2ARC devices can be striped, if desired, but 4TBs of L2ARC is probably beyond a reasonable amount with only 64GB of RAM.
I agree with others, test without L2ARC / Cache and see ARC hit rates suggest adding a L2ARC device. L2ARC is one of the vDev types that can be added or removed easily. Further, even if data does not make a difference for L2ARC, perhaps Metadata only in the L2ARC may make a difference.
There is a Resource and discussion about L2ARC in the link below. I’ve set the link to the post that includes a calculator using various parameters.
Unfortunately the Resources discussion thread is a bit long, and does have off topic posts. Perhaps someday we will trim it down.