I have a Mesh network of 3 Deco E4s in our house.  We have some blind spots, so am looking to expand (2 or 3 more).  Question is... what do I buy?  

1. More E4s?  Keep the network on the same units... but I expect the E4 is maybe getting to end of life now?

2. Upgrade the additional units to something else?  S4?  X10?

I have read in the forum that although I can mix and match in a Deco network, having different units can cause problems.  Is there an ideal "Upgrade-partner" to the E4?

If the recommendation is to upgrade, is it best to swap the Main plugged in Router E4 to the newer unit and move that previous E4 to the upstairs room we have the blind spot?

FYI.. I am on c60Mbs broadband at my house (4 bed).  We haven't experienced any real problems so far with the E4s.  Our main computer and TV are hard wired to the router, so we use the wifi for portable computing/tablets/printers etc.

Thanks in advance for feedback