Fallout 4’s location is straightforward, but modded restoration is not. A base .fos file can reference content from plugins, and F4SE can add a paired cosave. The safest archive therefore includes the full save directory plus enough information to rebuild the mod environment. A failed load after moving PCs is not automatically evidence of file corruption; missing plugins, changed load order, or an absent cosave can be the real cause.
Known Fallout 4 save-file structure
The researched filename or folder pattern is *.fos
*.f4se (when created by F4SE). File-extension context: .fos, .f4se The documented format notes are: Base Fallout 4 saves use .fos. F4SE-enabled installations can create companion .f4se cosaves; preserving only one side of a modded save may remove script-extender state.
Why an extension is not enough
A save filename can identify a likely container, but it does not prove the internal schema, game version, checksum rules, encryption state, or editability. A clean base-game save and a heavily modded save have very different restoration requirements even if both use .fos. SaveEditor therefore separates file identification from Tool compatibility and only labels a game-specific Tool Verified when its exact profile/version evidence passes the runtime gate.
Where the format is stored
PC save locations: Windows: %USERPROFILE%DocumentsMy GamesFallout4Saves | Linux / Steam Play: Steam Play: <SteamLibrary-folder>/steamapps/compatdata/377160/pfx/. Storefront context: Steam, Microsoft Store / Xbox app. The standard Documents path is the primary Windows reference. Microsoft Store packaging can differ, so locate the active edition before restoring a file. This matters because two releases can use different containers or companion files even when the visible slot name is similar.
Back up before inspecting or editing
Copy the complete Saves directory. On F4SE setups, keep matching .fos and .f4se files together and record the installed mod/load-order state. A read-only viewer is lower risk than a writer, but the live save should still be copied first. Cloud context also matters: Cloud behavior depends on storefront. OneDrive may also synchronize the Documents tree if Windows folder backup is enabled. Keep the original bytes available so any experiment can be reversed without relying on a cloud service or automatic backup.
What this reference does not claim
This page does not infer an undocumented checksum algorithm, encryption key, compression layer, or cross-store conversion from the file extension. Troubleshooting evidence: If an apparently valid save crashes on load after migration, verify missing plugins and script-extender dependencies before replacing more files. Also check whether Documents is redirected by OneDrive. Those questions require a verified parser/profile or a game-specific source, not a generic format guess.
Verification context: The standard Windows and Steam Play save locations were reviewed against current PCGamingWiki coverage. Format notes distinguish base .fos saves from optional F4SE cosaves.