I can only see battle isle phase 4, how do I get the wilderness missions?

NVM there's a start menu shortcut. Weirdly enough the .EXE of the expansion doesn't show up in the main game folder, or the GOG launcher.

Well, that's because there is no separate EXE for WM in this edition. There is single Incubation.exe which acts as an engine for the base game and for the expansion as well. And there is Launcher.exe which sets up correct launch parameters for the main EXE.
If you run Launcher.exe without parameters, it spawns ordinary Incubation.
If you run Launcher.exe -addon, it spawns Wilderness Missions.