def total_value(self):
    """Return the sum of the win, exacta, trifecta and first four values for this race"""
    
    total_value = 0.0
    for value in (self.win_value, self.exacta_value, self.trifecta_value, self.first_four_value):
        if value is not None:
            total_value += value
    return total_value