either 2 or 3. which are rather weak. We really need a page to explain that.
Oh wait, Combat mentions it but only has data for categories 1-5.
Cat 1 caps at 23498 hp, 2 is 26415, 3 is 29332, 4 is 35167, but 5 is 41001.
The easiest way to get cat 4 dragons is to mix opposing elements together(ice+fire, metal+terra, electric+nature, dark+light, or sea+war). Also, some sea/dark hybrids are cat 4. But you can't do that directly. A dragon with the light main type simply can't breed with a dark. But, if you breed a light/metal with a Terra/dark, that works fine. Also the level based breeds produce more powerful dragons. Mostly cat 4s, but the Hydra Dragon is a cat 5.