Find max and min date over multiple fields of the same record
If I understood you correctly, I'd use CROSS APPLY with VALUES to "unpivot" your fields and then a standard MIN and MAX can be used. Something like this:

DECLARE @T TABLE (ID int IDENTITY, f1 int, f2 int, f3 int, f4 int);

INSERT INTO @T (f1, f2, f3,f4) VALUES
(1, 2, 3, 4),
(5, 6, 7, 8),
(7, 8, 0, 1),
(2, 3, 6, 5);

SELECT ID, MIN(f) AS MinF, MAX(f) AS MaxF
FROM
    @T
    CROSS APPLY
    (
        VALUES (f1), (f2), (f3), (f4)
    ) AS CA(f)
GROUP BY ID
ORDER BY ID;


Or without CROSS APPLY:

SELECT 
    ID
    ,(SELECT MIN(f) FROM (VALUES (f1), (f2), (f3), (f4)) AS Fields(f)) AS MinF
    ,(SELECT MAX(f) FROM (VALUES (f1), (f2), (f3), (f4)) AS Fields(f)) AS MaxF
FROM @T
ORDER BY ID;


One more variation of the same theme. Aggregate within CROSS APPLY.

SELECT ID, MinF, MaxF
FROM
    @T
    CROSS APPLY
    (
        SELECT MIN(f) AS MinF, MAX(f) AS MaxF
        FROM (VALUES (f1), (f2), (f3), (f4)) AS Fields(f)
    ) AS CA
ORDER BY ID;


SQL Fiddle

All variants return the same result

+----+------+------+
| ID | MinF | MaxF |
+----+------+------+
|  1 |    1 |    4 |
|  2 |    5 |    8 |
|  3 |    0 |    8 |
|  4 |    2 |    6 |
+----+------+------+


The last variant has a better-looking execution plan for this simple table.