What is the model number introduced May 1999?
CREATE TABLE "comparison_table_of_hp_laser_jet_4000_mo" (
  "model" real,
  "introduction" text,
  "discontinued" text,
  "cpu_speed" text,
  "print_resolution_dpi_resolution_is_given_in_dots_per_inch_dpi" text,
  "print_speed_ppm" text,
  "standard_memory" text,
  "maximum_memory" text
);
SELECT MAX("model") FROM "comparison_table_of_hp_laser_jet_4000_mo" WHERE "introduction"='May 1999';