--===== This just creates demonstrable test data and is NOT part of the solution
    SET NOCOUNT ON
DECLARE @TestData TABLE(String VARCHAR(10))
 INSERT INTO @TestData (String)
 SELECT 'A012' UNION ALL
 SELECT 'A12' UNION ALL
 SELECT '12' UNION ALL
 SELECT '1' UNION ALL
 SELECT 'AB12' UNION ALL
 SELECT 'AB2'

--===== This is the solution for the known constraints on the data as posted
 SELECT Original =  String,
        Modified =  CASE
                        WHEN d.LastLetter = 0 THEN RIGHT('0000'+d.String,4)
                        WHEN d.LastLetter = 1 THEN LEFT(d.String,d.LastLetter)
                                                 + RIGHT('0000'+SUBSTRING(d.String,d.FirstDigit,4),3)
                        WHEN d.LastLetter = 2 THEN LEFT(d.String,d.LastLetter)
                                                 + RIGHT('0000'+SUBSTRING(d.String,d.FirstDigit,4),2)
                    END
   FROM (--==== Derived table "d" finds the interface between letters/digits
         SELECT String,
                LastLetter = PATINDEX('%[a-z][0-9]%',String),
                FirstDigit = PATINDEX('%[0-9]%',String)
           FROM @TestData
        )d

RESULTS:
=======
Original   Modified      
---------- ------------- 
A012       A012
A12        A012
12         0012
1          0001
AB12       AB12
AB2        AB02

--===== This just creates demonstrable test data and is NOT part of the solution
    SET NOCOUNT ON
DECLARE @TestData TABLE(String VARCHAR(10))
 INSERT INTO @TestData (String)
 SELECT 'A012' UNION ALL
 SELECT 'A12' UNION ALL
 SELECT '12' UNION ALL
 SELECT '1' UNION ALL
 SELECT 'AB12' UNION ALL
 SELECT 'AB2'


--===== This is the solution for the known constraints on the data as posted
DECLARE @MaxWidth TINYINT
    SET @MaxWidth = (SELECT MAX(LEN(String)) FROM @TestData)
 SELECT Original  = String,
        Modified  = LEFT(d.String,d.LastLetter) 
                  + RIGHT('0000'+SUBSTRING(d.String,d.FirstDigit,4),@MaxWidth-d.LastLetter)
   FROM (--==== Derived table "d" finds the interface between letters/digits
         SELECT String,
                LastLetter = PATINDEX('%[a-z][0-9]%',String),
                FirstDigit = PATINDEX('%[0-9]%',String)
           FROM @TestData
        )d

RESULTS:
========
Original   Modified           
---------- ------------------ 
A012       A012
A12        A012
12         0012
1          0001
AB12       AB12
AB2        AB02

SELECT	string Original
		,CASE	WHEN ISNUMERIC(string) = 0 
				THEN LEFT(string, PATINDEX('%[0-9]%',string)-1) 
				ELSE '' END					-- Characters first
		 + REPLICATE('0', 4-LEN(string))				-- Stuff with zeros
		 + SUBSTRING(string, PATINDEX('%[0-9]%',string), 4)		-- Add numeric remainder
		AS	Modified
FROM @TestData

--===== This is the solution for the known constraints on the data as posted
DECLARE @MaxWidth TINYINT
    SET @MaxWidth = (SELECT MAX(LEN(String)) FROM @TestData)
 SELECT Original  = String,
        Modified  = LEFT(d.String,d.LastLetter) 
      + RIGHT(
                REPLICATE('0',@MaxWidth)
              + SUBSTRING(d.String,d.LastLetter+1,@MaxWidth)
      , @MaxWidth-d.LastLetter)
   FROM (--==== Derived table "d" finds the interface between letters/digits
        SELECT String,
                LastLetter = PATINDEX('%[a-z][0-9]%',String+'0')
          FROM @TestData
        )d

SELECT	string Original		
	,CASE	WHEN PATINDEX('%[0-9]%',string) > 0 				
		THEN LEFT(string, PATINDEX('%[0-9]%',string)-1) 				
		ELSE RTRIM(string) END				-- Characters first		 
	+ REPLICATE('0', 4-LEN(string))				-- Stuff with zeros		 
	+ CASE	WHEN PATINDEX('%[0-9]%',string) > 0 
		THEN SUBSTRING(string, PATINDEX('%[0-9]%',string), 4)
		ELSE '' END					-- Numeric Trailer
	AS	Modified
FROM @TestData

--===== This just creates demonstrable test data and is NOT part of the solution
SET NOCOUNT ON
DECLARE @TestData TABLE(String VARCHAR(10))
INSERT INTO @TestData (String)
SELECT 'A012' UNION ALL
SELECT 'A12' UNION ALL
SELECT '12' UNION ALL
SELECT '1' UNION ALL
SELECT 'AB12' UNION ALL
SELECT 'AB2' UNION ALL
SELECT 'A' UNION ALL
SELECT '' UNION ALL
SELECT 'A14' UNION ALL
SELECT 'B2' UNION ALL
SELECT 'OBT' UNION ALL
SELECT 'YH3' UNION ALL
SELECT 'XX'


--===== This is the solution for the known constraints on the data as posted
DECLARE @MaxWidth TINYINT
    SET @MaxWidth = (SELECT MAX(LEN(String)) FROM @TestData)
 SELECT Original  = String,
        Modified  = LEFT(d.String,d.LastLetter) 
      + RIGHT(
                REPLICATE('0',@MaxWidth)
              + SUBSTRING(d.String,d.LastLetter+1,@MaxWidth)
      , @MaxWidth-d.LastLetter)
   FROM (--==== Derived table "d" finds the interface between letters/digits
        SELECT String,
                LastLetter = PATINDEX('%[a-z][0-9]%',String+'0')
          FROM @TestData
        )d