Four-year mission
Four-year missions were a type of long-term exploratory missions conducted by Starfleet.

In an alternate timeline, the  under the command of Captain Harry Kim was sent on a four-year mission from 2400 to 2404. His absence caused Captain Kim to miss several social events back home. ()

See also 
 Five-year mission
 Eight-year mission

Missions and expeditions
Starfleet