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: 
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.
**For X terminal emulators (eg. `$TERM` is `xterm`, `st`, etc.):**

```
$ tabs -N

```

**For virtual consoles (no X Server, only text console where `$TERM` is `linux`, etc.):**

```
$ setterm --tegtabs N

```

Where `N` is an integer number of spaces to be used for TAB.

**Sed way insead of `tr` which translates `SET1` to `SET2` of characters in `1:1` 'proportions':**

```
$ sed 's/\t/    /g' FILENAME
           ^^^^^
        spaces here

```