CODE:

private button_Click(object sender, EventArgs e)
{
	Button button = (Button)(sender);
	switch(button.Name)
	{
		case "button1":
			textBox1.Text =  "Test1";
			break;
		case "button2":
			textBox2.Text =  "Test2";
			break;
		case "button3":
			textBox3.Text =  "Test3";
			break;
		case "button4":
			textBox4.Text =  "Test4";
			break;
	}
} 

CODE:

private button_Click(object sender, EventArgs e)
{
	Button[] button = { button1, button2, button3, button4 };
	TextBox[] textBox = { textBox1, textBox2, textBox3, textBox4 };
	for (int i = 0; i < button.Length; ++i)
	{
		if (((Button)sender) == button[i])
		{
			textBox[i].Text = string.Format("Test{0}", i  + 1);
		}
	}
}