How To Manipulate String In Query And Create New Field

I'm very new to SQL server and can use some help. MyTable has ColumnA, which contains strings composed of 1 to 4 numeric characters (0 thru 9) followed by alphabetic characters. For example, "53ASDF". In my query, I need to create ColumnB, which takes the numeric prefix from ColumnA's string and prepends it with zeros, if necessary, to create a string of exactly 4 numeric characters. For example, I could get the following result:

ColA ColB
"6abc" "0006"
"457def" "0457"
"7232hij" "7232"

I have implemented a temporary solution using a CASE statement:
SELECT ColA, ColB =
CASE
WHEN ISNUMERIC(LEFT(ColA, 4)) = 1 THEN (LEFT(ColA, 4))
WHEN ISNUMERIC(LEFT(ColA, 3)) = 1 THEN '0' + (LEFT(ColA, 3))
WHEN ISNUMERIC(LEFT(ColA, 2)) = 1 THEN '00' + (LEFT(ColA, 2))
WHEN ISNUMERIC(LEFT(ColA, 1)) = 1 THEN '000' + (LEFT(ColA, 1))
ELSE ''
END
FROM MyTable


Because of additional complexities, I need to implement the solution with a loop instead of a CASE statement. Can someone please describe such a solution?

I'm very confused about how variables work in SQL Server, but made an attempt to implement a solution. Hopefully, someone can make corrections and describe how to use it with a SELECT statement. I would greatly appreciate any suggestions. This is what I started with:

DECLARE @ColBstring char(4)
DECLARE @num int
SET @ColBstring = ''
SET num = 1;
-- Get the numeric prefix from ColumnA's string
WHILE(isnumeric(substring(colA, 1, num)) = 1)
@ColBstring = (substring(colA, 1, num)
num = num + 1

-- Prepend the ColumnB string with zeros
WHILE(LEN(@ColBstring) < 4)
@ColBstring = '0' + @ColBstring


Thanks for any help,
Mike