i just got a tooth part of a crystal key from mysterious old man and i would like to know what is the key 4?

If you get the loop half you can use it with the teeth half to make a crystal key.

 

 

 

Then you can open a locked chest in northern tarverly. It's the house nearest the gate and the chest has this whitish outline aura thingy.

thank you

(not to mention the uncut dragonstone)

you get an uncut drag stone and sometimes 3 rune bars.