Appending multiple files into a single file

Let's assume you have file1.txt file2.txt file3.txt
Open the terminal and...

..$ cat file1.txt file2.txt file3.txt > file4.txt

VIM tricks:
*While you are in normal mode you can press "ZZ" to save and exit quickly
*You can go insert mode to below the current line with pressing "o" button
*If you want to delete current line quickly in normal mode, you should press "dd"