A split 7z archive isn't magic. Each part is just a chunk. Part 004 is not the "special" one — it's just the fourth piece. But without it, the chain breaks. However, sometimes part 004 can be recreated if you know the total size and the hash of the complete archive.
If you legally own Elden Ring , the DLC is a direct download through Steam for $39.99. Messing with shadowoftheerdtree.7z.004 suggests you are attempting to pirate a game that requires the base executable to be cracked. As of this writing, Shadow of the Erdtree Denuvo anti-tamper has not been publicly cracked. Many .004 files circulating are either: shadowoftheerdtree.7z.004
A: No. You need parts .001, .002, and .003. A single volume of a multi-part RAR/7z archive is like having only the fourth page of a 400-page book. A split 7z archive isn't magic