Good news everyone,
I have reimplemented the game in Godot 3 (from Godot 2). This is basically a complete rewrite. It has cleaner code and some minor improvement.
As such I have released it under the MPL2 and it is available on github:
https://github.com/gapophustu/4D-Minesweeper
The previous version is still available as as the legacy branch on steam.