If you lost your progress or want to start specifically at the beginning of Chapter 4 without playing through the first three chapters, you can download a "100% Complete" or "Chapter Start" save file.
A typical save file starting at Chapter 4 will have the following progress and unlocks: Story Missions Completed:
Note for Steam/Anniversary Edition players: The save file naming convention differs. On the PS2, files are .ps2; on PC (Scholarship Edition), they are .sav; on Mobile/Anniversary, they are cloud-based, but manual injection is possible.