CREATE TABLE abc
(num int)

insert into abc values (1010)
insert into abc values (111)
insert into abc values (99)
insert into abc values (0)

SELECT 'D'+ CAST(num AS NVARCHAR) FROM abc

SELECT 'D' + LEFT(CAST(num AS VARCHAR(4))+'0000',4) 
FROM abc

SELECT ISNULL('D' + LEFT(CAST(NULLIF(num,0) AS VARCHAR(4))+'0000',4),'0') 
FROM abc

SELECT CASE num WHEN 0 THEN '' ELSE 'D' END + 
    RIGHT(num*POWER(10, 3-FLOOR(LOG10(CASE num WHEN 0 THEN 1 ELSE num END))), 4)
FROM abc