You wire up the
TextChanged[
^] event and you can use
SubString[
^] to get the last 4 digits. For example:
private void textBox1_TextChanged(object sender, EventArgs e) {
if (textBox1.Text.Length >= 4) {
textBox2.Text = textBox1.Text.Substring(textBox1.Text.Length - 4);
}
}