If you have never installed v10.4.0 before, it will download the full installer (~1.6 GB). If you have a newer catalog (e.g., from v11 or v12), you cannot downgrade the catalog file. You must convert a backup catalog from the v10 era or start a new catalog.

Honesty requires addressing what you do in v10.4.0:

Adobe Photoshop Lightroom Classic 2021 (v10.4.0) is a desktop-based, non-destructive photo editor that organizes your workflow through specialized "Modules" Core Workflow Modules

"Stop," she said.