IMP pairs. The actual sequence after 4♠ was X P P lead facedown, XX by S on the principle that no-one was going to deprive him of his rightful call. West was a pro, whose double can be explained by him being about 5 whiskies into the evening. North had no idea whether either his 1N or X were correct, but didn't want to be left out of the entertainment.
Both 4♠ and 5♥ were making, so South's redouble was one of the more costly actions I've seen in the history of bridge (especially if you count N's double as one of the consequences).