This is an even simpler option:

WITH VTE AS
  (
   SELECT
    *
   FROM
    (
      VALUES (3),
        (25.75),
        (12),
        (125.8),
        (1000),
        (1857.5)
    ) V (I)
  )
SELECT VTE.I, FORMAT(I*10, '0000') AS LeadingZeros FROM VTE;