It’s a common elemental theme we see across stories, fantasies, etc. It seems like it is an element that should be part of that pantheon but maybe there is a scientific explanation of why it isn’t? Or is it simply, they did not have enough understanding of it at the time those 4 were suggested.
That I can’t really answer, as there’s no real in universe measuring stick for bending power. Some people clearly just have more oomph to their bending though, though I can’t say if any bender could reach that peak level through training, or if they have to be born with a certain level of talent/power. Zuko certainly shows that bending can improve through training. In flashbacks we see him struggle with basic bending techniques, and little natural ability.
Conversly, in that same flashback, Azula makes a good argument for natural talent, being a bit of a prodigy at it. Korra takes that argument even further, being able to bend 3 elements when she’s like 5 or something.
If we think of the time the Gaang assaulted the earth palace to talk to the earth king, we see them fight through pretty much the entire Dai Li/palace guard to get to the king. These would all be highly trained earthbenders, some of the kingdoms elite. But they essentially get chewed through by 3 kids.
So basically, much like real life, training can help a bender improve. A lot. But also like real life, sometimes people have just a ridiculously high natural ability that pushes them above the herd. Aang, Toph, Katara, Azula, these are the Jordans, Gretzkys, Kobes of the bending world. So both nature and nurture.