@FrederickP 

This installation would work, but it may not get you an optimal WiFi coverage. 

I would try something different, but it depends on how large your house is, is attic room as large as first floor or just one room, where M4 is located on the first floor and type of building materials used for attic floor.

With favorable (for my recommendations) assumptions, that is what I would do.

I'll get single additional M5 unit, and using wall mount M5 holder that can be bought separately, will mount M5 on the ceiling of the first floor, right under attic room. My assumption is attic floor is made of wood, not concrete.

I would reposition M4 which is on the first floor a bit closer to M5, ideally having them in the line of sight.

This should get me good WiFi backhaul link between M4 and M5, as well as adequate WiFi coverage in the attic room.

Alternative option: I'll place that M5 unit inside attic room, and lay Ethernet cable between M4 on the first floor and M5 in the attic, to connect them both by that cable. Hopefully it would not be much of a hassle to drill few holes in the attic floor to pass Ethernet cable through, for the benefit of getting really good WiFi coverage in the attic room.

@Alexandre. Thank you for you help and reply. Have a nice day !

@FrederickP 

Just to clarify about P9: you'll need two Deco P9 to have them use powerline backhaul.

This is good idea, in fact, but another option doing similar thing could be a pair of generic powerline adapters and additional single Deco M4 or M5 unit. Adapters will create and Ethernet link for this new Deco unit in an attic to use as a backhaul. 

Finally, to exhaust all available options: if you have TV cable outlet in attic, you should research MoCA adapters. If they can work for you, it is substantially better solution that powerline.