Two classic Tour cities, Carcassonne and Foix, host the fourth stage: 181.9 km, 2,700 meters of elevation gain for a stage that is often tackled in the opposite direction, as an approach to the Pyrenees.

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The route follows the roads of the Cathar lands and then those of Ariège, there isn't much flat terrain and riders must contend with four categorized climbs. The climbing begins with the Col de Bedos (4th category, 3.4 km at 4.4%, at km 48) followed by the Col du Paradis (3rd category, 5.9 km at 4.1%, km 65). Then comes the Col de Coudons (2nd category, 10.8 km at 5.5%, at km 195) and finally the Col de Montsegur (2nd category, 6.9 km at 6.6%, at km 146.7): this latter climb stands out as the true watershed moment, destined to settle the matter between the day's attackers and the teams of the more resilient sprinters who might decide to seize the opportunity.

After skirting the ruins of the historic Montsegur castle - site of a historic siege against the Cathars with the tragic end of 222 people burned alive at the foot of the fortress, in what is today called Prat dels cremats - the finish line is positioned at the foot of another castle, that of Foix, a splendid medieval jewel.