@Jeff1976 

The official TP-Link answer will be "any other Deco model is compatible," but in your specific case with WiFi5 Deco mesh I would recommend to consider the following:

Deco M5

Deco M4R

Deco S7

Deco S4

Out of all of them Deco M5 is going to be the most compatible, hardware and software wise. I recommend M5 based on my experience. My Deco mesh is two M9 Plus and two M5s I bought after deploying M9 Plus set.

Also, if your new Deco addition will connect to your Deco mesh wirelessly, not through Ethernet backhaul, consider Deco E4.

Deco E4 has 100Mbps Ethernet ports. All other Deco models have gigabit Ethernet. For that reason, Deco E4 is often on sale. Ethernet ports do not play much role if nothing is Ethernet wired to that E4 and if E4 itself connects to Deco mesh wirelessly.

  @Alexandre. 

I finally bought 2 DECO E4 connected by cable to the network.

No need for Gigabit, only Wifi!

THANKS !