Smallest of Greenland, Borneo, Madagascar, New Guinea: Madagascar

Ranking by land area (descending):

1. Greenland – 2,166,086 km² (world's largest island)
2. New Guinea – 785,753 km²
3. Borneo – 748,168 km²
4. Madagascar – 587,041 km²

Madagascar is the smallest of the four. (All are >500k km²; next smaller is Baffin Island at 507k km².)