@RonaldRSC 

See this document that discusses how Deco units must be connected to use Ethernet backhaul: General questions about Ethernet Backhaul feature on your Deco

If that document does not help, you'll need to tell to where Deco #4 is connected with Ethernet cable and perhaps draw a simple network diagram.

  @Alexandre. 

Thanks for your reply, your document was helpful, I created a star network now and that helped.

At first I connected all three ethernet stations to my internetsupplier delivered modem/switch, after adding a separate switch in between the master and the slaves, the problem was solved.

I was a bit confused because I migrated from 3 Deco M5 stations and they were connected as I tried in the first place and they were working well, so I thought that should be the case for the X50 as well.

Thanks for your help.

Cheers

Ronald