Golden Sun: Dark Dawn

In Golden sun is the main characters you play mostly Matthew, Karis and Tyrell? is there a 4th guy who is a water adept?

Yes. There are two water adepts that I've found so far,

Spoiler

Amiti and Rief

If this is anything like the other GS, you'll probably get 8 characters. 2 adepts of each element.

Other three character is...

Spoiler

Sveta, Eoleo and Himi.

There are eight adepts in total, two of each element.