@davidecit 

Hi, I don't think it makes any difference when choosing anyone of them.

I believe the 4th Deco is still Deco P9. Or are you planning to add a Wi-Fi Backhaul only Deco?

If it is still P9, you could just make sure to stay away from high-power Home Appliances, like Washing Machines, Micro Oven and try to avoid a surge protector.

For other non-Powerline Backhaul Deco, consider placing the fourth unit within sight of a nearby upstairs P9, such as near the staircase or along the path to downstairs rooms where Wi-Fi is mostly needed.

Thank you very much and best regards.