Answer
The four Horta buildings on the UNESCO World Heritage List are: the Hôtel Tassel, the Hôtel Solvay, the Hôtel van Eetvelde, and the Maison & Atelier Horta. These buildings showcase Horta's innovative use of iron, glass, and ceramic tiles, which revolutionized architecture at the turn of the 20th century and established the Art Nouveau style in Belgium.