The 4th factor would be 3. You would find this out through a factor tree. If you don't know the original number when you start you look at your numbers. A factor of 1 usually goes with the highest number, assuming that the number 6 is the highest factor, that would be the original number. As 2 is the other known factor than we know that 3 has to be the last factor, because 2 x 3 = 6. It cannot be the number 12 because then there would be 6 factors not the needed 4.