template <typename pins_conn>
class lcd
{
  public:
    lcd() = default;

    void attach()
    {
      con.attach();
    }

    constexpr bool is_4bits() const
    {
      return con.is_4bits;
    }

    pins_conn con;
};

struct conn_4bits
{
  void attach()
  {
    // Attach 4 pins
  }

  void send_cmd(int cmd)
  {
    // Send 4 bits command
  }

  int d4, d5, d6, d7;
  int is_4bits = 1;
};

struct conn_8bits
{
  void attach()
  {
    // Attach 8 pins
  }

  void send_cmd(int cmd)
  {
    // Send 8 bits command
  }

  int d0, d1, d2, d3, d4, d5, d6, d7;
  int is_4bits = 0;
};

template <typename lcd_con>
void do_something_with_lcd(lcd<lcd_con>& lcd)
{
  // Do something
}

int main()
{
  lcd<conn_4bits> lcd_4bits;
  lcd<conn_8bits> lcd_8bits;

  // Setup
  lcd_4bits.con.d4 = 1;
  lcd_8bits.con.d0 = 2;
  // Etc

  lcd_4bits.attach();
  lcd_8bits.attach();

  if (lcd_4bits.is_4bits())
  {
    // Do something...
  }

  do_something_with_lcd(lcd_4bits);
  do_something_with_lcd(lcd_8bits);

  return 0;
}