text: It took $36.4m (£26.7m), trouncing Melissa McCarthy's latest comedy, Tammy, which took $21.2m (£15.6m).
  "This ranks as one of the lowest fourth of Julys ever," said Paul Dergarabedian, from box-office tracker Rentrak.
  Takings were down 44 per cent compared to last year.
  US Independence Day weekend is traditionally marked by the release of "tentpole" movies - from Back to the Future to Terminator 2 and Men In Black.
  Last year's major release was animation Despicable Me 2, which went on to take $368m (£214m), making it the fourth-biggest movie of the year in the US. But this year, there was no major release to coincide with the holiday.
  Michael Bay's Transformers, the fourth in the series, has a cast led by Mark Wahlberg.
  It has received generally poor reviews from critics, as has Tammy.
  The only other major new release was the horror film Deliver Us From Evil, starring Eric Bana. It opened in third place with $9.5m (£6.9m).
  The box office will be hoping for a boost next weekend with the release of Dawn of the Planet of the Apes, starring Andy Serkis and Gary Oldman. It is expected to be one of the summer's big hitters.
target: Transformers: Age of Extinction has held on to the number one spot at the North American box office on an unusually quiet 4 July weekend.