I'm wondering why certain stringed instruments (fiddle, viola, cello, mandolin) have neighbouring strings a fifth apart whereas others (bass fiddle, bass guitar, most pairs on guitar) are fourths. Is there a reason for this that can be explained in less than two hundred words? Is one kind of tuning considered more useful for one kind of musical function, and vice versa?

Actually, let me rephrase that in terms of what I really want to know:

I've just bought a cello. Besides learning to play melodies and countermelodies on it with a bow, I want to make it serve as a fake bass fiddle for bluegrassy type backup, playing triads and plucking the strings. But a cello is tuned in fifths, and a bass fiddle in fourths. Is there any reason why I should - or shouldn't - consider retuning the cello into fourths when I want to pretend it's a bass?

Thanks, Marion