@Lexivienna 

Hi, please refer to this link:

https://www.tp-link.com/en/support/faq/2248/-Q2

But If you plan to wire all the Deco units together since both P9 and M4 are gigabit, It does not make a huge difference which one is set up as the main Deco.

Thank you very much.

  @Lexivienna better don't, they are not actually compatible as they said. Had many signal issues with M9 Plus combined with M4R