C# 如何把Picturebox图片付给bitmap

[Super龍卷風]

我定义了一个bitmap，想把picturebox4的图像付给它，然后在上面作图，再赋给pictureBox1.可是出了问题：

private Bitmap pic1; 
pic1 = pictureBox4.Image;//这一句出错了
//错误9无法将类型“System.Drawing.Image”隐式转换为“System.Drawing.Bitmap”。存在一个显式转换(是否缺少强制转换?)
Graphics g1 = Graphics.FromImage(pic1);
Pen mypen = new Pen(Color.Red);
g1.DrawLine(mypen, tempx_pre, tempy_pre,tempx,tempy); 
g1.Dispose();
pictureBox1.Image = pic1;

[whowhen21]

pic1 = (Bitmap)pictureBox4.Image;

[bdmh]

Bitmap bmp = pictureBox1.Image as Bitmap;

[骑着蜗牛去爬山]

Bitmap bmp = (Bitmap)pictureBox1.Image;

[Super龍卷風]

再咨询一个，如何让这个pic1清空啊？