private const string format = "n3";
private readonly IFormatProvider provider = new NumberFormatInfo { NumberDecimalSeparator = ".", NumberGroupSeparator = "," };

private Double Parse(string value)
{
    return String.IsNullOrWhiteSpace(value) ? 0.0 : Double.Parse(value, provider);
}

private void OnSomethingChanged(object sender, EventArgs e)
{
    // Игнорируем изменения, сделанные нашим кодом, а не пользователем
    if (sender is TextBox && !((TextBox)sender).Modified)
    {
        return;
    }

    double one, two, three, four, x;

    if (textBox4.Modified)
    {
        one = Parse(textBox1.Text);
        two = Parse(textBox2.Text);
        four = Parse(textBox4.Text);

        x = four / (one - two * two);

        textBox3.Text = x.ToString(format, provider);
        textBox4.Modified = false;
    }
    else
    {
        one = Parse(textBox1.Text);
        two = Parse(textBox2.Text);
        three = Parse(textBox3.Text);

        x = (one - two) * three;

        textBox4.Text = x.ToString(format, provider);
    }
}