Why do bass guitars have only 4 strings?
The bass among other things gives the music and rhythm depth. The more strings the higher the possibility in pitch; to high and the bass loses its original purpose of depth and beat. That being said, there are now basses that are five string instruments.
For starters, they reduced the amount of strings because of the thickness of the strings. It is a lot more comfortable to play. Yet you can find just as many 5 string bass's as 4 strings. There are also many 6 strings on the market. I have also seen up to 8 string bass's on the available for sale, as well as private guitar builders who have made them with as many as 14 strings. Although the were played almost as an upright.
*Correction: the number of strings was never 'reduced', the electric bass was developed from the upright bass (aka Double Bass or Contrabass) shortly after the introduction of the electric guitar. Jazz bassists were finding it hard to be heard over the top of the highly amplified electric guitars so followed the "if you can't beat them, join them" approach and created Bass guitars, with the same scale length, strings and tuning as a Double Bass since that was what they knew. (the similarities in standard tuning between the lowest strings of a guitar and the 4 strings of a bass is a coincidence as far as I am aware)
The Double Bass is a member of the string instrument family, a group of instruments (violin, viola, cello and double bass), descended from the viola da gamba and similar old bowed instruments, which have developed to have four strings (and occasionally, more recently, five), as the best solution to the many problems surrounding their construction.
Since Bass Guitars, with their solid construction, have no issues with the weight-strength-tension capability ratio, they have been able expand the number of strings to give players a more extensive range. Although Pastorius never needed more than four so I don't see why anyone else should :P