According to the publisher, it incorporates all the previous material plus 2500 new entries, and is available as a PDF download:
Bryan Martin, MusBac, MA
University of Toronto
From: Association for Recorded Sound Discussion List <[log in to unmask]> On Behalf Of Thomas Stern
Sent: March 29, 2018 12:28 PM
To: [log in to unmask]
Subject: [ARSCLIST] Großes Sängerlexikon
Großes Sängerlexikon (Kutsch & Riemens) - does the 4th edition include updates to the 3rd, or simply incorporate the 2 supplemental volumes? anyone know if they plan to issue a cd-rom ?