1) I never said Hylia was a fourth goddess.
The Power Hylia has was almost certainly given to her by the goddess' themselves. It is referred to as the "Light Force" I believe by vaati (been awhile since i play MC)
2) Demise is obviously shown to be equal in power to hylia by the expedient that...
I feel i have to point out, Ikana is shown to be in essence a dead man's land. A place of no hope, a realm of suffering and nightmare of tortured souls. It is also well known that kings who once believed in gods tend to rebel and do everything in their power to slur their name. In Ikana's case...