Problem description: After switching to FDT4 M3, I pointed the workspace straight back at its old location, and of course a fair number of errors showed up that needed fixing. One of them, though, kept telling me “The Project type has to be set.” in the SDK settings (FDT compiler) — yet there’s nowhere in that screen to change the so-called “Project type”.
Solution: Select the project, right-click - Flash Project - Set project type… set it, hit OK, and you’re good.