Honestly, I haven't used mdadm in a long time, but if memory serves (and it didn't change) I don't think mdadm support adding caching drives. (though there maybe alternatives to add a caching drive)
I just want to point out that if your drives are SATA drives or better which I suspect they are given they are 4TB drives combined with likely a motherboard with a 6GB SATA bus. I'm pretty sure those SATA drives will smoke (saturate) your 2.5GB Ethernet. You're more likely to be limited by your processors ability to encode the video stream depending on what is powering it.
My buddy streams as many as 8 streams at once on his dedicated desktop PC running Plex on a single 8TB SATA drive. His limiting factor was his processor's ability to encode the streams on the fly and upload bandwidth.
My QNAP runs my Plex server, but I may have 2 users tops and that is once in a blue moon.