Efficient Algorithm to print count and possible string combinations of a string by retaining order and length of charactersined

Problem: Given a input string as "0000" and following input scheme for the zeroes, print the possible combinations by retaining order of the characters.
Input Scheme:
A "0"
B "00"
C "000"
D "0000"
Example: "AAAA" is one string with length and order of characters retained