The fifth envelope contains the elements required for The Factories, Episodes 13, 14 & 15. Opening up the envelope will give you access to everything you need for the next 3 games. But I will reveal the information needed for each of those episodes one at a time, under the individual spoiler tags below.
Chapter 5 – The Factories
End of Chapter 5
That is the end of Chapter 5. The game carries on in Chapter 6: The Mine.