As I understood it--so take this with a grain of salt--motivation is the key difference between levels 4 and 5.
Both sorts of kids can learn pretty much whatever you put in front of them, but level 5s are the sort who are just driven to learn virtually all the time. Level 4s are less self-motivated but just as capable. And level 4s can become level 5s when working on something they care about. Their pet projects transport them into level 5 territory.
As a side note: I always warn against the notion that GT kids don't need to be taught. By definition, they do learn things more quickly and easily than the average kid, and some things they certainly piece out by themselves without instruction. But I worry that this "they teach themselves" idea plays into neglectful teaching situations and lack of challenge. Some GT kids really go like mad on their own, but not all. And even these self-motivated kids are almost certain to hit a point at which they need some help of some sort from an experienced teacher. Granted, that may not happen until graduate school, but I think it will inevitably occur if a kid keeps growing and learning. And most GT kids need teaching long before grad school if they are to keep growing.
Sorry to soapbox you.
I just think that this is a potentially dangerous myth about GT kids, and it worries me, so I always try to say something when the subject comes up.
Carry on!
