A preliminary hearing on the murder charges against Christian Karl Gerhartsreiter, who for years masqueraded as Clark Rockefeller, will probably take place in January, his lawyer said yesterday. Gerhartsreiter, who is being held outside Los Angeles, is accused of killing his landlady’s son, John Sohus. Brad Bailey, one of Gerhartsreiter’s two lawyers, said his client will return to Alhambra Superior Court Oct. 17 to set the date for the preliminary hearing. During that hearing, the burden will fall on prosecutors to persuade a judge that the case against Gerhartsreiter, who has pleaded not guilty, should move forward. Sohus and his wife, Linda, disappeared in 1985. Her remains were never found, but California authorities believe she is dead.