I know there's been progress on the Deco M4R (which I think is very close to the S4). Thread here: OpenWrt support for TP-Link Deco M4R - #83 by bobthebuilder
However, to install OpenWRT, we need access to the telnet console. I tried using the TP-Link M4R firmware with telnet access found in the thread above using my Deco on the recovery page. Unfortunately, my Deco wouldn't even allow me to install it (perhaps it was due to an issue with the model number). FCC ID of the Deco S4 shows it's an M4R v2 - that's why I thought I'd give it a try.
If anyone has a TP-Link firmware for the Deco S4 where we can obtain telnet access; or has figured out a way to build the firmware so we can load it natively on the S4, I'd love to hear from you. 