What is the channel number that has a branding of PTV 4 Laoag?
CREATE TABLE "ptv_stations_nationwide" (
  "branding" text,
  "callsign" text,
  "ch_num" text,
  "power_k_w" text,
  "station_type" text,
  "location_transmitter_site" text
);
SELECT "ch_num" FROM "ptv_stations_nationwide" WHERE "branding"='PTV 4 Laoag';