Error- License does not match

[GIS_EntESRI]

I am creating a cluster of Red Hat Enterprise Linux 7.4 Virtual Machines with ArcGIS Server 10.5 installed and licensed with EDN license. The Sever Site adds 3 VMs but when I try to add a 4th one it displays an error stating 

"The license of ArcGIS Sever at 'vm4arcgisserver' does not match with the license of the site running at 'https://:vm1arcgisserver.esri.com:6443/arcgis/admin'. All the machines in the site are required to have the same license."

I am curious whether it is a licensing issue which is limiting me to add more than 3 VMs or there are some configurations needed to be modified? 

[DerekLaw]

Hi GIS_Ent Esri,

First, be advised that the Esri EDN program has been replaced by the new ArcGIS Developer Subscription program. I believe EDN subscribers were migrated to the 'Professional' tier of this new program.

FYI: Pricing | ArcGIS for Developers 

Second, an EDN license is meant for developer testing/prototyping and you would typically only install and configure a single machine ArcGIS Server site. Licensing a multi-machine Server site is not really following the terms of use. If you need to license additional machines for testing, please discuss your requirements with your local Esri account manager/Distributor.

Hope this helps,