Creating new column with data from other columns

Hello all my data looks like this 

Col 1 | Col 2 | Col 3

1       |           |

2       |           | 

3       |           | 

         | 4        |
         | 5        |
         | 6        |
         |           | 7
         |           | 8
         |           | 9

and I want to make a new column (keep the originals ) that looks like this as well as keeping the values in thier original rows. 

Col 4

1

2

3

4

5

6

7

8

9

thank you