Combile (Append) 2 or more columns

I've multiple columns of data which I want to put in to 1 single column.

EX: Columns A,B,C Data is appended in to D.

A B C D
1 5 9 1
2 6 10 2
3 7 11 3
4 8 12 4
13 5
14 6
7
8
9
10
11
12
13
14