Oracle 18c

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WITH your_table
AS (
    SELECT *
    FROM (
        SELECT 9999 AS primary_key, 1 AS column_1, 2 AS column_2, NULL AS column_3, NULL AS column_4 FROM DUAL
        UNION ALL
        SELECT 9999, NULL, NULL, 3, 4 FROM DUAL
        UNION ALL
        SELECT 9999, 5, 6, NULL, NULL FROM DUAL
        )
    )
, UNPIV AS (
    SELECT PRIMARY_KEY, 'column_1' AS COLUMN_NAME, column_1 AS COLUMN_VALUE
           , ROW_NUMBER() OVER(PARTITION BY PRIMARY_KEY ORDER BY column_1) AS RN
    FROM your_table
    WHERE column_1 IS NOT NULL
    
    UNION ALL
    
    SELECT PRIMARY_KEY, 'column_2' AS COLUMN_NAME, column_2 AS COLUMN_VALUE
           , ROW_NUMBER() OVER(PARTITION BY PRIMARY_KEY ORDER BY column_2) AS RN
    FROM your_table
    WHERE column_2 IS NOT NULL
    
    UNION ALL
    
    SELECT PRIMARY_KEY, 'column_3' AS COLUMN_NAME, column_3 AS COLUMN_VALUE
           , ROW_NUMBER() OVER(PARTITION BY PRIMARY_KEY ORDER BY column_3) AS RN
    FROM your_table
    WHERE column_3 IS NOT NULL
    
    UNION ALL
    
    SELECT PRIMARY_KEY, 'column_4' AS COLUMN_NAME, column_4 AS COLUMN_VALUE
           , ROW_NUMBER() OVER(PARTITION BY PRIMARY_KEY ORDER BY column_4) AS RN
    FROM your_table
    WHERE column_4 IS NOT NULL
    )
SELECT PRIMARY_KEY, 
       MAX(CASE WHEN COLUMN_NAME = 'column_1' THEN COLUMN_VALUE END) AS COLUMN_1,
       MAX(CASE WHEN COLUMN_NAME = 'column_2' THEN COLUMN_VALUE END) AS COLUMN_2,
       MAX(CASE WHEN COLUMN_NAME = 'column_3' THEN COLUMN_VALUE END) AS COLUMN_3,
       MAX(CASE WHEN COLUMN_NAME = 'column_4' THEN COLUMN_VALUE END) AS COLUMN_4
FROM UNPIV
GROUP BY PRIMARY_KEY, RN
ORDER BY PRIMARY_KEY, RN

PRIMARY_KEY	COLUMN_1	COLUMN_2	COLUMN_3	COLUMN_4
9999	1	2	3	4
9999	5	6	null	null