@rammiz 

I have Deco M9 Plus at my house. I think you may not have full understanding of the TP-Link Deco mesh concept. Let's start with it. 

Every Deco M9 Plus is same hardware. There is no dedicated M9 Plus "router" and "extenders." When you set your Deco mesh, you pick first M9 Plus from the set you bought, any M9 Plus, and after it is configured it becomes Main Deco (equivalent to router). The rest Deco M9 Plus units from that set and any other Deco M9 Plus you add later will be Satellite Deco (equivalent to extender).

More to that, if your Main Deco experiences hardware failure and dies, you can take any Satellite Deco from those you already have, promote it to Main Deco, and you will have your Deco mesh up and running again. With one less Deco unit, of course, but not totally dead. This is what I really like about Deco mesh, comparing to other brands of WiFi mesh that have dedicated router.

TP-Link sells Deco M9 Plus in sets of 2 and 3. You can't buy single new Deco M9 Plus, I learned that when I wanted to expand my 2-unit set with one more Deco M9 Plus: I couldn't.

If you need four more Deco units, you'll have to buy 2-unit set and one more 2-unit set. If you need three - buy 3-unit set. 

You said you have 4 Deco M9 Plus units in your Deco mesh. You must have bought them as two 2-unit sets, I don't see any other way. If what you bought is described as "full M9 Mesh Router + a 3 pack of the Deco M9 Extenders" on the box they came with - please make photo of that box and share it.

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How many more Deco M9 Plus units do you need to expand coverage, assuming each new one will be Satellite Deco (extender): 3 or 4?

The reason I am asking, you seem to assume that when you buy Deco M9 set, one of the units will be a dedicated router and you may have to put it aside. It will not.

Also, what do you use to check signal strength and coverage at different places of your house (or business?) and what made you decide you need to add more Deco M9 Plus to your Deco mesh?