input: Write a python function to convert currency using given exchange rate and input amount.
  assert currency_converter('USD', 'EUR', 0.85, 100) == '100.0 USD = 85.0 EUR'
  assert currency_converter('EUR', 'GBP', 0.9, 200) == '200.0 EUR = 180.0 GBP'
  assert currency_converter('JPY', 'USD', 0.0091, 10000) == '10000.0 JPY = 91.0 USD'
output: def currency_converter(from_currency, to_currency, exchange_rate, amount):
      converted_amount = amount * exchange_rate
      return f"{amount:.1f} {from_currency} = {converted_amount:.1f} {to_currency}"