@Yoann 

>How to force the number 3 to connect to number 2?

There is no configuration setting in Deco to tell it which Deco to connect to. It appears Decos prefer to connect to Main Deco, as long as they can see WiFi signal from it, even if it is weak.

TP-Link Support says how Decos connect to each other will be improved in future firmware, but they don't know when this will happen.

There are at least two workarounds for your Deco mesh layout you could try.

1. Connect Deco #3 and Deco #2 with Ethernet cable.

This will force Deco #3 connect to Deco #2, because Ethernet backhaul takes priority.

Alternatively, you could try with the pair of powerline adapters, it should work if Deco #2 and Deco #3 power outlets are on the same circuit breaker.

2. Make Deco # 2 your Main Deco

Obviously, that will require wiring Deco #2 to your ISP router, but perhaps you could move ISP router closer to where Deco #2 is? 

When Deco #2 is Main Deco, Deco #1 and Deco #3 will prefer to connect to it. 

There is a risk that Deco #4 might decide to connect to Deco #2 (Main Deco) instead of Deco #3, but if you can make Deco #2 your Main Deco it could be worth to try. If it is not working, you can undo the changes.

@Alexandre. Thank you for all your advice :)

It's strange, i looked several times during the day, and now, my 3 additional deco M4 are directly connected to the first one (connected to internet box) ...

So i bought 4 deco for NOTHING !

The purpose of this system is to avoid cables and extend wifi ... i can't use cables in my home :(

I can't understand why, in my 3 floors house, with a Deco M4 on each floor, even the one at the top of my roof is connected to the one at the bottom ... there is 2 deco m4 between, and they are useless ...

TP-Link, what are you waiting for ? When are you gonna optimize your software ?

Thank you Alexandre :)