Last 4 dragons Crono was teaching in being a protectors, warriors and master there elements within

Hilda, the Huntress Dragoness
Element: Fighting/Soul+
Born and part of a group of huntress know for there bravery and skill. Hilda when comes the fighting will mark herself with special paint which will help with her abilities and to show fear in her foes. Hilda goes to Crono to learn not only elements but to be a leader for her clan.

Zeusa(Rai), The Unique Dragon
Element: Dust, Glass, Muta, and Sonic
Rai or going by the name of Zeusa is one of the strongest know Origin Element type dragons(Dragon with the first 4 elements) and not afraid to show that. Reason why he wants to be called Zeusa or why he training with Crono but there maybe some idea from Crono and the Elders. Zeusa is being training to respect and understand others as he always had a fact of being strong cause of his type and Zeusa maybe because of Crono name. Zeusa does show respect and will to listen still doesn't change his way.

Vamp, the L&D Dragon
Element: Light and Dark
Vamp was born not only with one rare mix type but also unique marks as well which makes him feel different. Vamp has strong powers on both sides but can go to each other but thanks to Moon he has got better. Vamp training his with Crono the fine his way in this way.

Nina, the Tech Dragon
Element: Eletric and Metal
One of the only dragon to know Crono without being part of his training at first until one of the Elders told her to. Nina helps the other get used to the new place, gear and vehicles and even help making some of the stuff too. Nina trains with Crono to learn her hidden powers and in time be someone very special as well.