in dragon city do dragons sometimes start out at level four

You can only remove a dragon if you have a free slot in a habitat that that dragon can be placed in.

I like to level my quest dragons to 40 as soon as I can comfortably afford to, and use the heroic to get all of the quest kills. Then you can click a Release button to move it to a habitat. Each dragon has to be at least lv15 for the 1st stage with the level requirements working their way up to 30 eventually at a cost of 1.7m food (565k x 3 dragons). To recall a dragon from the Dragonarium, tap on the building and search for the dragon you want.You also cannot sell a stored dragon from the dragonarium, you first need to move it to a habitat. Stored dragons do not produce gold, also the player cannot feed, train, empower, send them breed, battle nor retrieve a piece of tower but you can still recall a stored dragon.

'Too many dragons? Store your dragons in this new Deus´ masterpiece and keep collecting! Note that while your dragons are stored in the Dragonarium they will not produce gold, and you will not be able to use them either in the PvP leagues or in the Tournaments.' Each dragon has to be at least lv15 for the 1st stage with the level requirements working their way up to 30 eventually at a cost of 1.7m food (565k x 3 dragons).

In dragon city do dragons sometimes start out at level four