CREATE TABLE table_8764 ("Callsign" TEXT, "Frequency" TEXT, "Band" TEXT, "On-air ID" TEXT, "Purpose" TEXT)

What is 4zr's callsign?
SELECT "Callsign" FROM table_8764 WHERE "On-air ID" = '4zr'
SELECT "Callsign" FROM "table_8764" WHERE "On-air ID" = '4zr'