Everybody knows the story of the ill-fated RMS Titanic, which struck an iceberg on the 14th of April, 1912, and sank, leaving more than 1,5 thousand people dead. Titanic is also considered one of the best known four funnel liners in history, along with the RMS Lusitania, which was destroyed during the WWI.

The primary purpose of funnels on steamships were to allow smoke, heat and excess steam to escape from the boiler rooms. As liners became larger, more boilers were used. The number of funnels eventually became symbolic of speed and safety, so shipping companies even sometimes added false funnels to give an impression of power.

Thus, the Titanic's fourth funnel was actually fake. There was a popular belief that it (besides making the ship look bigger) made the vessel faster.

More Info: en.wikipedia.org