What is the fourth triforce on the hylian shield?

Its just there for decorative reasons. However Фаны used to believe that was the 4th part of the 'Tetraforce'(just the Triforce with a треугольник in the middle space). Pretty sure it has been removed since Wind Waker because of this.

its just there for a decretive disighn

I don't know. Unless the trolls at Nintendo planned Skyward Sword that long ago, then the extra piece could be the Goddess.(It does have the bird pattern...

Skyloft.

I agree, i heard long назад that they were planning skyward sword so i think that it represents Hylia.

4th= EPIC POWER!!!!!!!!!! HAHAHAHAHAHAHAHAHAHAHAHAHA!!!!!!!!!