Question: When I am in `vim` I can change the tab size with the following command:

```
:set ts=4

```

Is it possible to set tab size for `cat` command output too?

Answer: 
The first command here *emulates* the formatting you see in `vim`. It intelligently expands tabs to the equivalent number of spaces, based on a tab-STOP (ts) setting of every 4 columns. 

```
printf "ab\tcd\tde\n" |expand -t4   

```

Output 

```
ab  cd  de

```

To keep the *tabs* as *tabs* and have the tab **STOP** positions set to every 4th column, then you must change the way the environment works with a tab-char (just as vim does with the `:set ts=4` command) 

For example, in the terminal, you can set the tab **STOP** to 4 with this command;

```
tabs 4; printf "ab\tcd\tde\n" 

```

Output 

```
ab  cd  de

```
Based on the above answers and examples, it would seem that the actual command the OP wanted is...

`cat somefile | expand -t4`

This works for me on Red Hat 6.4.