Find an equation that uses all 9 digits (1-9)
It is extremely inefficient to do this randomly; you could end up checking the same numbers again and again. For a more structured approach, you could use itertools.permutations:

from itertools import permutations

for nums in permutations("123456789"):
    a = int("".join(nums[:4]))
    b = int(nums[4])
    c = int("".join(nums[5:]))
    if a * b == c:
        print "{0} * {1} == {2}".format(a, b, c)
        break # optional - there's more than one answer!




Also, note that:

found=False
while not found:
    ...
    if int(four_digit_num)*single_digit_num==int(multiple):
        found=True


would be neater as:

while True:
    ...
    if int(four_digit_num) * single_digit_num == int(multiple):
        break