E.S.C. wrote
Is it possible to connect a M5, M4 or another type of Deco unit, to my M9 plus 3 pack I have now?
@E.S.C.
I have 2-pack M9 and single M5, they work together just fine.
A word of advise: if you plan to use WiFi backhaul with additional Deco units, it makes sense to buy more M9. Otherwise you will be losing the primary benefit of M9 when it comes to new units: dedicated backhaul. It makes the difference.
In case you plan to wire new units with Ethernet cables, running M4, M5, M9 together in Deco mesh is totally OK.
This is how much difference dedicated wireless backhaul makes, based on non-scientific tests in my household, with ISP speed at up to 350Mbps. Running speedtest from Android smartphone connected to Deco units:
Main Deco M9 wired - 350Mbps
Satellite Deco M9 wireless, using dedicated backhaul - 350Mbps
Satellite Deco M5 wired - 350 Mbps
Satellite Deco M5 wireless, does not have dedicated backhaul - 170-200Mbps