A 4-string bass is usually tuned to E, A, D & G--one octave below the 4 lowest strings of this
[DOC] [TLE] Bass Guitar Tunings | Bass Gear | StudyBass [PAR] The standard tuning for a four-string bass from lowest pitch to highest ... B  E  A 
 D  G. This tuning has one lower pitched string added to the standard 4-string 
tuning giving you 5 lower ... a 4-string bass, but with a second string doubling 
each of those four strings.