The longest stage also presents the greatest overall elevation gain; for this reason the fourth stage can rightly be defined a key stage. The circuit leading out of Naturns/Naturno starts with a straight uphill, the first KOM of the day is located slightly later, when the group will be crossing the 1706-meters-long Hofmahdjoch/Passo Castrin tunnel, the highest spot of the 2021 Tour of the Alps. The long descent will carry the riders to Val di Non, leaning towards Dimaro. After the flat-out downhill, the road will begin to rise again towards Kaiser Karl der Große Pass/Passo Campo Carlo Magno: almost 1000 meters gain allocated over 14 km. The following climb will be the Selle Giudicarie, near Tione. However, the strongest athletes will probably decide to play the deciding card on the brand-new ascent of Castel Condino towards Boniprati: 10 km of vertical ascent ending 7km far from the Pieve di Bono finish line.