prova così

codice:
select valore,sum(quanti) as volte from 
(select campo1 as valore,count(campo1) as quanti from tabella group by campo1
union all
select campo2 as valore,count(campo2) as quanti from tabella group by campo2
union all
select campo3 as valore,count(campo3) as quanti from tabella group by campo3
union all
select campo4 as valore,count(campo4) as quanti from tabella group by campo4
) as tab group by valore