Appending columns one after the other

Hi please refer to eg below...This is just a test file have to do such stuff on an actual file which has lots of data...

See i have col1,col2,col3,col4

now i want to append coln2 data immediately where col1 ends,col3 data after coln2 data in col1 only,col4 data in col1 after col3 data and so on ....

Col1 Col2 Col3 Col4
1 4 7 10
2 5 8 11
3 6 9 12


Output should be a single column apended one after the other

1
2
3
4
5
6
7
8
9
10
11
12

..Hi thanks in advance if any of you'll can help me asap..