As per our comments, Mouse_Down would look something like this:
private void uxNumber_MouseDown(object sender, MouseButtonEventArgs e)
{
if (e.LeftButton == MouseButtonState.Pressed)
{
int num = Convert.ToInt32(((GradientButton)sender).Text);
if (this.uxPIN.Text.Length < 4)
uxPIN.Text += num;
else
SystemSounds.Beep.Play();
}
}