@Papaey 

I would recommend to rely on X20s and only use M4 and E4 as Satellites in places where X20 signal does not reach.

This is the process I would recommend:

1. Add all three X20s as Satellite Deco to your Deco mesh: How to add an additional unit to your Deco network

2. Promote one of Deco X20 to become Main Deco: Changing the Main Unit for your Mesh Network

3. Power off all Deco E4 and M4. Place X20s where you want them to be. Check WiFi coverage.

4. If WiFi coverage is weak in some places, power on M4s (as many as you need) and move to places where they will provide additional coverage.

5. After you figured how many M4/E4 you need/want to keep, remove the rest from your Deco mesh (you may have to turn on those you want to remove, to make sure they are reset properly). Follow "Reset a Deco unit" section from this document: How Do I Reboot or Reset My System?

  @Alexandre. 

Thanks sir.