column. therefore, the 1 needs to be in the fourth column. now there is only one possible place for the remaining 1.
in the first column, the 2 can only be in the second row, because of reasons similar to the one above. next we note that the 2 in the fourth column can only be in the bottom row, and the last 2 has only one available position left. the 3s and 4s can now be filled in rather easily. 