For ones asking themselves why are people takling about the 4th attribute...

New Sylph hero is considered to be the 4th spirit. There are 4 spirits in total, one for each attribute (Earth Spirit - str, Ember - agi, Storm - int).

Not all the spirits are released yet, so some people are expecting the new fairy hero to be the last spirit (I personally don't think that this is true).

This is why I think that we are not receiving a new attribute: There are already ~38 heroes for each attribute and a lot of them would need to change it to make all attributes even, so ~29 heroes per attribute, that would make everything completely imbalanced. Maybe Sylph is gonna be an original hero based around all attributes, or about her 4th attribute, but that is probably not going to happen.

The 4th attribute would maybe be morale, maybe stamina, who knows. Maybe it will give spell amp. instead of int, armor instead of agi or nerf everyone's magic resistance but make this attribute give a small % of it.