How do I add a line of text to the end of a file

I am using RHEL.  If I have a file a.a, and it contains the following:

first item
second item
third item

... and I want to add a line at the end of the file, and perform something like

concatenate  a.a  "fourth item"

and the result would look something like

first item
second item
third item
fourth item

(Note, this will requiree a CR)

What command would I use to perform this