Graphics g = e.Graphics;
// 圆角半径
int cRadius = 1;
// 要实现 圆角化的 矩形
Rectangle rect = new Rectangle(0, 0, panel4.Width - cRadius, panel4.Height - cRadius);
// 指定图形路径, 有一系列 直线/曲线 组成
GraphicsPath myPath = new GraphicsPath();
myPath.StartFigure();
myPath.AddArc(new Rectangle(new Point(rect.X, rect.Y), new Size(2 * cRadius, 2 * cRadius)), 180, 90);
myPath.AddLine(new Point(rect.X + cRadius, rect.Y), new Point(rect.Right - cRadius, rect.Y));
myPath.AddArc(new Rectangle(new Point(rect.Right - 2 * cRadius, rect.Y), new Size(2 * cRadius, 2 * cRadius)), 270, 90);
myPath.AddLine(new Point(rect.Right, rect.Y + cRadius), new Point(rect.Right, rect.Bottom - cRadius));
myPath.AddArc(new Rectangle(new Point(rect.Right - 2 * cRadius, rect.Bottom - 2 * cRadius), new Size(2 * cRadius, 2 * cRadius)), 0, 90);
myPath.AddLine(new Point(rect.Right - cRadius, rect.Bottom), new Point(rect.X + cRadius, rect.Bottom));
myPath.AddArc(new Rectangle(new Point(rect.X, rect.Bottom - 2 * cRadius), new Size(2 * cRadius, 2 * cRadius)), 90, 90);
myPath.AddLine(new Point(rect.X, rect.Bottom - cRadius), new Point(rect.X, rect.Y + cRadius));
myPath.CloseFigure();
g.DrawPath(new Pen(Color.Red, 1), myPath);