Merging files with cat

Display the contents of a file: cat file1.txt
Display on screen all three files like they were merged: cat file1.txt file2 file3.liz
Merge all three files in one file called file4.txt: cat file1.txt file2 file3.liz > file4.txt [Notice: Executing the command above doesn't show anything on the terminal. Inspect file4.txt instead]