The line art for this is several years old. Originally I called it 'Elemental Dragons' but there's not really a clearly defined 'earth', 'air', 'fire' (ok, the red's definitely fire so ignore that bit) or 'water' so it's just 'Four Dragons' instead.

Blue dragon (Arian bloodline) - air, water
Red dragon (Korth bloodline) - fire
Black dragon (Deryos bloodline) - earth, water
Green dragon (Ssithar bloodline) - earth