code: public function setLastDigits( $lastDigits )
      {
          $lastDigits = trim( $lastDigits );
  
          if (strlen( $lastDigits ) !== 4) {
              throw new InvalidArgumentException( sprintf( 'Last digits should contain the last 4 characters in the card number (%s)',
                  $lastDigits ) );
          }
  
          $this->lastDigits = $lastDigits;
  
          return $this;
      }
docs: @param  string $lastDigits
  
  @return $this