Shark Tank investor Kevin O'Leary, whose estimated fortune stands at $400 million (£316 million), has branded the growing push for a four-day workweek 'the stupidest idea I have ever heard,' arguing that the traditional concept of a set working week is already dead in the post-pandemic digital economy. The 71-year-old Canadian businessman, widely known as 'Mr. Wonderful,' made the remarks during an appearance on the Fox News programme Outnumbere…