Block IV Tomahawk is the current generation of the Tomahawk family of cruise missiles. The BGM-109 Tomahawk family began life in the 1980s as sub-sonic, low-flying nuclear strike weapons, before being developed into long-range RGM/UGM-109 conventional attack missiles. They’re most frequently launched from submarines and surface ships, and have been the US Navy’s preferred option for initial air strikes in Iraq, Libya, et. al. Britain has also bought Tomahawk missiles, and launches them exclusively from submarines.
Block IV is the latest variant. It adds innovative technologies that improve combat flexibility, while dramatically reducing the costs to buy, operate, and support these missiles. That’s why the Block IV program, under US Navy PMA-280, has been one of the USA’s defense acquisition success stories over the last decade.