Error code: 1222 the used SELECT statements has a different number of columns

SELECT  PsngrInit,PsngrLNam,SeatId,TourId
FROM passenger
INNER JOIN booking
ON passenger.PsngrId=booking.PsngrId
WHERE booking.PortionId='Melb-Geel'
UNION
SELECT SeatTypId
FROM seattype
INNER JOIN booking
ON booking.SeatId = seattype.SeatId