(1) x = 0, x = 4 is the boundary point of piecewise function, which can be written as the definition domain of any function.

(2) The opening, vertex and symmetry axis are all aligned with the complete parabola y = x? -4x+3, so the vertex is (2,-1), just like the opening is upward. Although (0,3) and (4,3) are also extreme points, they should be understood as endpoints or extreme points.