By Sara C. Medina

Captain Kirk and Mr. Spock in the streets of San Francisco? The surprise is there was no surprise. Not a San Franciscan eye batted while the makers of Star Trek IV: The Voyage Home used a hidden camera to record man-in-the-street reaction to Kirk's glowing wine-red suit or Spock's white robe and ear- covering headband. Just didn't seem strange enough to stand out in the city by the bay. ''One lady even approached me after watching the shoot,'' reports Leonard Nimoy (Spock), ''and said, 'I thought you were a monk or a priest.' '' In the latest, $23 million installment of the TV-turned-movie series, slated for December release, the whole Enterprise gang, led by William Shatner, journeys back in time 300 years to Starfleet's first home base, San Francisco, to save planet earth from an alien life form that had its beginnings in the 20th century. (No, it's not Cheez Whiz.) Perhaps in honor of Star Trek's upcoming 20th anniversary, the new film purports to be the closest so far in form and feeling to the original TV shows. Nimoy directed, as he did the third movie, and he claims, straight-faced as always, ''It has the most down-to-earth structure--pardon the pun--of all four films.'' Oh, those Vulcans. Aren't they just the ones to always be making fun?