The 4HE High-Fusion Reactor was a revolutionary piece of engineering invented by Starfleet in the late 25th century. It utilized a high-energy reaction between helium-4 and it's antimatter counterpart. The standard Helium-4/Antihelium-4 reaction contained 45% more energy than the Deuterium/Antideuterium reactions. That's about 261 petajoules of energy in a normal 1 kg matter/antimatter reaction.