Q:

Change format of text file

I have a file with many lines of tab separated data in the following format:
1 1 2 2
3 3 4 4
5 5 6 6
...

and I would like to change the format to:
1 1
2 2

3 3
4 4

5 5
6 6

Is there a not too complicated way to do this?  I don't have any experience with using awk, sed, etc. 
Thanks

A:

If you just want to group your file in blocks of X columns, you can make use of xargs -nX:
$ xargs -n2 < file
1 1
2 2
3 3
4 4
5 5
6 6

To have more control and print an empty line after 4th field, you can also use this awk:
$ awk 'BEGIN{FS=OFS="\t"} {for (i=1;i<=NF;i++) printf "%s%s", $i, (i%2?OFS:RS); print ""}' file
1 1
2 2 

3 3
4 4 

5 5
6 6 
        # <-- note there is an empty line here

Explanation

On odd fields, it print FS after it. 
On even fields, print RS. 
Note FS stands for field separator, which defaults to space, and RS stands for record separator, which defaults to new line.  As you have tab as field separator, we redefine it in the BEGIN block. 

A:

This is probably the simplest way which allows for customisation
awk '{print $1,$2"\n"$3,$4}' file

For a line between 
awk '{print $1,$2"\n"$3,$4"\n"}' file

although fedorquis answer with xargs is probably the simplest if this isn't needed
As Ed pointed out this wouldn't work if there were blanks in the fields, this could be resolved using
awk 'BEGIN{FS=OFS="\t"} {print $1,$2 ORS $3,$4 ORS}' file

