Four star admiral

Four star admiral (also known as full admiral or simply admiral) is a rank of flag officer in naval services and space forces, most notably Starfleet. Four star admiral is a senior flag officer rank, a grade higher than a vice admiral and a grade below fleet admiral and other admiral grades in most rank structures. This rank is roughly equivalent with the authority of a general of military forces.

In military parlance, is it acceptable and normal to refer to a four star admiral simply as "admiral" in most forms of address.

In a redesign of the admiral's service uniforms of 2366, an insignia of four gold pips enclosed in a rectangular gold border was worn by four star admirals, showing the origin of the "four star" name. This insignia continued into later uniform designs of the 2370s. 

United States Navy

 * David Farragut (19th century)

Federation Starfleet

 * Alynna Nechayev (2370s)
 * William Ross

Background
The United States rank scheme that Starfleet was adapted from is also the origination of the insignia, background information, and alternate name for this rank. In the United States armed forces, full admirals of the US Navy wear a pin insignia of four stars on their uniforms and hold an officer grade equivalent to a general of other services.