The Galaxy Primes (Ace SF, 27291)

They were four of the greatest minds in the Universe: Two men and two women, all Psionic Primes, lost in an experimental spaceship billions of parsecs from home. And as they mentally charted the cosmos to find their way back to Earth, their own loves and hates were as startling as the worlds they encountered... Here is E. E. Smith's classic science fiction novel -- one of the greatest space operas of all time!