private void button4_Click(object sender, EventArgs e)
{
RegistrationForm reg_form = new RegistrationForm();
reg_form.ShowDialog();
}
private void linkRegister_LinkClicked(object sender, LinkLabelLinkClickedEventArgs e)
{
RegistrationForm regform = new RegistrationForm();
this.Hide();
regform.ShowDialog();
this.Show();
}