A good 4x4 should feel stable, corner cut cleanly, and stay controllable during fast turning. The biggest difference between beginner and 'competitive' 4x4s is how they handle speed: strong internals, good magnet strength, and reduced lockups make a huge impact. Premium models often add features like UV coating for grip and advanced internal “track” designs to improve stability and alignment.
However, some cubes go a little too far and end up reducing their quality (E.g. The Double track AoSu is often seen as better than the triple track as the triple is a little bit too magnetic for many cubers styles of turning).