Just now I made a sh5 mission in a small harbor then I went into sh4's mission editor and went to the same harbor and put in a minefield(with a stupidly high density) and subnets. Then I copied the relevent info from the mission txt for the sh4 mission into the sh5 txt.
The mission editor would crash if I tried to open that mission, as expected. But the game loaded fine, same with the mission. But I looked around and couldn't find any mines or nets
Yet one more thing they didn't finish.