Combine like rows based on substring of one column?

I have a table of parts that looks like this:

Part	Part Num	Thing1	Thing2	Thing3	Thing4
Door	10105322	abc	abc
Door	10105323	abc	abc
Door	10105324	abc	abc
Door	84625111	abc	abc	abc
Door	84625118	abc	abc	abc
Door	84625185	abc	abc		abc
Door	56897101	abc	abc

The part number is always 8 characters. For many parts, the first 6 characters are the same, and the last 2 are different. Lines where the first 6 characters of a part number are the same and all rows that have the same values in Thing1/Thing2/Thing3/Thing4 need to be combined, and the part number becomes 6 characters. (row 1/2/3 in the above table)

Lines where the first 6 characters are the same but the values in Thing1/Thing2/Thing3/Thing4 are NOT identical in all rows need to remain unchanged and the part number remains 8 characters. (row 4/5/6 in the above table)

Lines where the first 6 characters are unique need to remain unchanged and the part number remains 8 characters. (row 7 in the above table)

The desired result would look like:

Part	Part Num	Thing1	Thing2	Thing3	Thing4
Door	101053	abc	abc
Door	84625111	abc	abc	abc
Door	84625118	abc	abc	abc
Door	84625185	abc	abc		abc
Door	56897101	abc	abc

Answer

Use COUNT() window function:

WITH cte AS (
  SELECT *,
         COUNT(*) OVER (PARTITION BY Part, LEFT(PartNum, 6), Thing1, Thing2, Thing3, Thing4) counter1,
         COUNT(*) OVER (PARTITION BY Part, LEFT(PartNum, 6)) counter2
  FROM tablename
)
SELECT DISTINCT
  Part,
  CASE WHEN counter1 > 1 AND counter1 = counter2 THEN LEFT(PartNum, 6) ELSE PartNum END PartNum,
  Thing1, Thing2, Thing3, Thing4 
FROM cte;

See the demo.