U4 (Bit 4). You cannot downgrade to a lower binary (e.g., U1, U2, or U3) once your device is on U4.
The Samsung Galaxy J4 (SM-J400F) Go to product viewer dialog for this item. samsung-j400f-u4-flash-file
Typically corresponds to Android 9 (Pie) or early Android 10 builds for this binary level. U4 (Bit 4)