Hello, please tell me where can I buy PLATE SCREWS for ALEMBIC EPIC 4 strings ?. Thank you very much for your help.

Flores,
 
You don't say where you are, but, you can get them from Alembic.

per the FAQ: For truss rod covers and backplates, the inserts are very small 2-56 size
 
so, the screws are 2-56 size, that is a number two machine screw with 56 threads per inch. you can check the length against one you already have, unless they are all gone.