"Only the Avatar can master all 4 elements" is said for a reason. Only the avatar can bend more than one element. Normal benders can work within their elements, to 'create' new types of bending. There are a few examples of this in Avatar: Legend of Aang, as well as Legend of Korra.
Water-bending: Since blood mostly consists of water, Katara is able to (but doesn't like to) Blood-bend, in that she can actually control a person's movements, and I think also kill them.
Fire-bending: Azula and Uncle Iroh give some stunning displays of Lightning-bending. Since both fire and electricity in the most basic of scientific terms are energy, if a fire-bender were to highly concentrate that energy they put into producing fire, then they could in theory produce electricity.
Earth-bending: Most metals have traces of chemical earth elements. That is why in Legend of Korra, Beifong and her team of Metal-benders actually exist. For metals that have lover traces of chemical earth elements, it is harder, and for metals like Platinum, that have virtually no chemical earth elements, it is near impossible for her and her team to bend.
I can't remember any subdivisions of air bending, but I'm sure Tenzin will go all BAMF on us and show us some cool tricks.