The description only says gog 4 - gog 5, but it doesn't explain much, espcially that there are 5 episodes in total in the full installer, so it cannot possibly be a post-release patch. Or is it?
Been a while since you posted, but I'll answer anyways in case you're still wondering, or someone else might be.
The patch that is "4 - gog 5" updates game installs that are currently at episode 4 to episode 5, for those that already had episode 1-4 installed, and didn't want to download the whole game installer again.
The full game installer will always be the most up to date version.
Most of the time, patches will update from a certain older version to the newest version.
In the case of episodic games like this, the "newest" patch is not necessary if you are doing a new install of the game.