@piknew 

This is, indeed, strange: I checked half an hour ago in Deco app and it had the option to choose Main Deco. Just now I wanted to make a screenshot of Deco app to show you where it is, and option is not there! 

I rebooted smartphone and I've got it back in the menu.

Try rebooting your smartphone, it might help.

  @Alexandre. Sorry, maybe i was not clear - I am not saying that the option to change is missing. I meant that I am not able to choose Deco M4 as main device. Please see my config as for now - there is M5 (set as main) and M4 as sattelite. I am going to remove M5 from my setup but I need to change main deco to M4 first. But as you can see - Deco M4 is even not displayed to be selected:

  @piknew 

Something might be up with Deco app or TP-Link cloud. Try the following steps, in that order, and after each step check if Deco app will allow to select M4 as new Main Deco.

1. Unplug Ethernet cable from M4, bring it to where M5 is and keep there with strong WiFi backhaul signal.

2. Reboot smartphone.

3. Restart Deco mesh using "Reboot Network" from the Deco app.

4. Add second M4 to Deco mesh, keep it with WiFi backhaul. 

  @Alexandre. None of above worked. However, I tried additional point - simply I go to details of M5 and selected "remove", I was warned that my coverage will be down - but I have confirmed. Then... I was not able to see my network in the application. But - I have seen my SSID, it was providing access to internet. I went to webpage of my M4 (1st and 2nd), checked and... One of them was automatically assigned as main Deco.

After some time (10-15 minutes) - my Deco application (on Mobile/Tablet) start showing correct structure of network with two M4 Decos.

And... After selecting "change main deco" - I see both. So, it seems that changing of main deco is using priorities of devices (M5 has higher priority). So, I can change main deco from M4 to M5, but IT IS not possible to do this in opposite way (M5 -> M4).

I belive that "recommended" priorities given by TP-LINK are somehow forced priorities.

  @piknew 

I have Deco mesh that consists of M9 Plus and M5, with M9 Plus as Main Deco. I can choose M5 as Main Deco, even if it has lesser priority in TP-Link recommendations.

Still, I think your theory has its merit, but with additional clarification: that it applies specifically to M4 and small number of other older Deco models. See this from Deco app:

It is possible that only M4/.../S4 Deco models won't be shown in Main Deco selection. That's why I can see M5, but you can't - M4. A legacy restriction still sitting in Deco app code, perhaps.