Currently, employees have the legal right to request flexible working, but there is no obligation on companies to agree. That balance of power is to be shifted, with companies instead legally obliged to offer flexible working from day one except where it is “not reasonably feasible”.

Seems this isn't about a 4 day week specifically, but about employers being required to give their reasons when they reject flexible working requests. The telegraph is just using it as an example of something an employee could request.