"In space, no one can hear you scream." But you'll be hearing screams in theaters if producer/director Walter Hill has his way: He's signed on with 20th Century Fox to continue as co-producer (with David Giler and Gordon Carroll) of both "Alien 3" and "4." And he's negotiating with Ridley Scott to direct "3"-Scott helmed the original for the trio-which has a storyline that sounds suspiciously like political allegory. Hill told us that although Sigourney Weaver wouldn't reprise her role as the heroic Ripley in "3," "she hasn't been written out-she'll have a prominent role in `Alien 4.' " The script for "3," now being penned in Vancouver by William Gibson, is based on a story by Hill and Giler. According to sources, the storyline involves an attempt by the corporate backers of space exploration to harness the fearsome aliens as a weapon against the enemies of capitalism. Also plotted: Earth scientists successfully splicing alien and human genetic material, resulting in an army of controllable human/alien hybrids that are unleashed upon a socialistic, third world space colony. Byline: Sheldon Teitelbaum