@Integralist 

The answer is "it depends." In fact, you might actually end with stronger WiFi signal but slower download speed. You can read more about it if you google "range extenders can cut bandwidth in half"

Before you spend money on additional hardware, I would suggest you temporarily move Satellite M4 from "opposite room" to "corridor." It is recommended, for Satellite nodes that are not wired and use WiFi to connect to Main node, to stay in the middle between Main node and where is your location.

After you moved M4 to corridor, check if signal from M4 to Main node is strong there (at least two bars reported in Deco app), and if signal on your WiFi device such as smartphone, when in the "opposite room" and connected to M4 in the corridor is strong, too.

If it does, you have found better location for your Satellite M4.You might not need to buy another unit.

That all changes if you can wire Satellite Deco to Main Deco with Ethernet cable - then, the best location for Satellite Deco would be as close as possible to where you are.