DTSgp wrote

Currently I have 2 units M4, thinking to add another 1-2. What's the recommendation model besides M4?

  @DTSgp 

The most affordable would be E4, but only if you plan to use it as a Satellite Deco with wireless backhaul. E4 has 100Mbps Ethernet ports, if used with wired Ethernet backhaul its maximum Internet speed will be limited by that, not exceeding 100Mbps. Deco backhaul is discussed here: https://www.tp-link.com/us/support/faq/1794/

Other than that: Deco S4 (Deco S4 and M4 have the same features and spec), Deco M5, Deco M1300 (same as M5, just different name). 

There is also Deco S7, but it is listed as "coming soon." It has three Ethernet ports, otherwise it appears similar to M4/S4.

You may consider M9 Plus, but these are more expensive and to use their benefits you'll need to deploy them properly. Otherwise, they won't be much better than much cheaper Deco M5.