Register MouseClick event of grid and use following code.
private void dataGridView1_MouseClick(object sender, MouseEventArgs e)
{
DataGridViewRow dr = dataGridView1.SelectedRows[0];
textBox1.Text = dr.Cells[0].Value.ToString();
textBox2.Text = dr.Cells[1].Value.ToString();
textBox3.Text = dr.Cells[2].Value.ToString();
textBox4.Text = dr.Cells[3].Value.ToString();
}
And add the following line in your load event
dataGridView1.SelectionMode = DataGridViewSelectionMode.FullRowSelect;