@RJT463 

Yes, additional Deco M4 will work together with existent M4 you have, no problem. With Deco, there are no "primary" and "satellite" Deco units different in hardware and design. They all are the same and interchangeable.

See How many Deco units can I add to my Deco mesh network?

You will be adding new Deco units to existent Deco mesh, through Deco app. The process must be familiar to you, but it is also described here: Adding an additional unit to your TP-Link Deco network

Also, if you need to add less than three, you can get 2-pack or single unit and they can be added to your existent Deco mesh, with no issues. 

Finally, it is really good idea to keep same type of hardware for WiFi mesh, and this is what you are doing. Yet, if you have place where you need to wall mount or ceiling mount Deco unit, consider Deco M5. It'll work with M4 just fine, but its compact configuration with additional wall mount holder you can buy from the Amazon makes M5 perfect for narrow corridors or if you need to attach Deco unit to the ceiling.

If you don't have such requirements, stay with M4 - just add more, as you plan.

@RJT463 Thanks for your advice. I made a mistake in the original question. I have P9 system at the moment. Do i have to buy more P9s or can I add to the whole system with M4s. M4s are half the price of P9s.

@RJT463 

You can add M4. They will be communicating with P9 by WiFi, not by powerline obviously as M4 don't support powerline connectivity, but other than that they'll work just fine together with P9 in your Deco mesh, using M4 as additional Satellite units. Keep P9 as Main Deco.

If you are adding Deco units other than P9 to your Deco mesh, your options are Deco M4 and Deco M5. Both are similar in performance, M4 is tower-type and M5 is more compact design. Decide which works better for you by look and by price. You can mix them, too. For example on a TV stand compact M5 may look better and elsewhere you can have M4s.

Also, if all new Deco units will be connected to your Deco mesh wirelessly, not by Ethernet cable, check Deco E4. By WiFi performance it is practically the same as M4, but E4 has 100Mbps Ethernet ports (not gigabit) and won't deliver more when it is connected by Ethernet cable. For that reason, Deco E4 are often on steeper discount than M4, yet if using as Satellite Deco without Ethernet connection they are as good as M4.