Could you advise which will be the best chose from Synology 4-Bay NAS – DS918+ Vs DS418play Vs DS418 Vs DS418j for small rendering farm 2-4 PC (Vray) or maybe something different from Synology family?
For so specific task you will need a NAS with virtual machine support. That narrows Synology range down to plus series. DS918+ will let you install a Linux on the virtual machine and later you can set up your rendering farm using instructions below.
How to build your own render farm https://arstechnica.com/information-technology/2014/05/how-to-network-lots-of-dumb-computing-muscle-in-a-fast-efficient-render-farm/4/
A more serious solution would be intel core or Xeon CPU NAS likes DS3617xs. But rendering uses video resources, so if you are not afraid of AMD processors – TS-677 series will let you install graphics card for even better rendering performance.
Or you can consider cloud rendering options:
Cloud rendering :