@Divit 

There are good reasons for you to stay with X20, and to buy another X20 as add-on. 

If you recently bought Deco X20, you should expect them to last at least 5 years. A lot could change in these 5 years. For example:

1. Even if someone absolutely guarantees you that any mobile device today will seamlessly roam between M4 and X20, there is no guarantee new smartphone, laptop, tablet, etc. you buy 2-3 years from now can.

2. It is possible that few years from now there may be mobile device no longer supporting WiFi5. If you will buy one of those, they won't roam from X20 to M4.

3. You'll get better support from TP-Link if you stay on same hardware. TP-Link releases firmware updates, and you may have seen on this forum people ask for features such as whitelist, WPA3, DOH. Some of these may come in new releases. Enabling some of new features may require all Deco units in the mesh upgraded with new firmware.

It is possible that in a few years M4 will be deemed "End of Life/End of Sale" and firmware with new features won't be released for it. X20 is newer line of Deco devices and it should be getting firmware upgrades for longer. Which means, in the future, with M4 in your Deco mesh, you might not be able to use some of new features which came with the latest X20 firmware upgrade.

@Alexandre. Alright, I'll look into buying an X20 then, if only to futureproof my network. Thanks for your help :)