OK thanks - out of interest could i add some of the M5 flater smaller nodes?

bdlp wrote

 OK thanks - out of interest could i add some of the M5 flater smaller nodes?

  @bdlp 

Yes, you can. Same thing: you can buy single M5, two-nodes or three-nodes M5 set. They can be seamlessly added to your existent Deco mesh as Satellite Deco.

From all different Deco models TP-Link sells, you may consider the following as addition to your Deco E4 mesh: Deco E4, Deco M4, Deco S4, and Deco M5.

  @bdlp 

One of the reasons I went with Deco WiFi mesh instead of a different brand, is because there is no Main Node with different hardware. All Deco nodes have same hardware.

Taking your Deco mesh as an example. You plan to have 5-nodes Deco mesh. If Main Deco experiences hardware failure, you can promote any other Satellite Deco to become  Main Deco. See Changing the Main Unit for your Mesh Network. You'll keep running 4 nodes WiFi mesh until you decide what to do with defective unit.

For a brand that has Main Node different from Satellites, if Main Node fails, you are losing WiFi until you replace it. That makes the difference for me.

Also, even if Deco E4 becomes End of Sale, and one of your units fails after warranty, you can buy different model of TP-Link Deco and use it to replace failed unit. The selection of possible replacement models is substantial.