with t1(col1, col2, col3,col4) as (
  values ('1',  '2',  '3',  '4'),
         ('1',  '2',  '3',  NULL),
         ('1',  '2',  NULL, '4' ),
         ('1',  '2',  NULL, NULL),
         ('1',  NULL, '3',  NULL),
         ('1',  NULL, NULL, '4' ),
         ('1',  NULL, NULL, NULL),
         ('1',  '2',  '3',  '4' ),
         (NULL, '2',  '3',  NULL),
         (NULL, '2',  NULL, '4' ),
         (NULL, '2',  NULL, NULL),
         (NULL, NULL, '3',  NULL),
         (NULL, NULL, NULL, '4' ),
         (NULL, NULL, NULL, NULL)
),
t2(row_id, col1, col2, col3, col4) as (
  select ROW_NUMBER() OVER() as row_id, col1, col2, col3, col4
  from t1
),
t3(row_id, col_id, value) as (
  select row_id, 1, col1 from t2
  union all
  select row_id, 2, col2 from t2
  union all
  select row_id, 3, col3 from t2
  union all
  select row_id, 4, col4 from t2
)

select LISTAGG(value, ', ') WITHIN GROUP (ORDER BY col_id)
 from t3
 group by row_id;

'1, 2, 3, 4'
'1, 2, 3'
'1, 2, 4'
'1, 2'
'1, 3'
'1, 4'
1
'1, 2, 3, 4'
'2, 3'
'2, 4'
2
3
4
NULL