REGEXP REPLACE backreference in a function

If i use backreference in LPAD function, in the result the padding characters number always the same (2, but why?), not depends on the padded string length:

SELECT REGEXP_REPLACE('aaa 123 bbb 45 ccc 6 ddd', '([0-9]+)', LPAD('\\1', 4, '0'));

aaa 00123 bbb 0045 ccc 006 ddd

How can i reach this result:

aaa 0123 bbb 0045 ccc 0006 ddd