If you occasionally find yourself in a Word document or Excel spreadsheet applying the same formatting to different parts of the file, then F4 will come in handy as it repeats the last command.
This is especially useful if the formatting you are applying is only accessible in a dialog box. Rather then opening the dialog box each time, just press F4. For instance, you might be applying custom borders to all the total rows in a large spreadsheet or turning on the Keep with Next option for many paragraphs in Word.
F4 works in Word, Excel and PowerPoint and it repeats the last command, whatever it is. So you could use it to delete rows in an Excel spreadsheet, for instance.
Sometimes as I am using F4 to apply formatting at various places in a larger document I’ll notice an error in the text. I’ll delete a few words to fix the problem or type something to improve the flow. Then I’ll go back to what I was doing and press F4 expecting it to apply formatting. Surprise! F4 repeats the very last command so it’ll delete or insert text, depending on what I did to fix the problem. Not a big deal, of course, as I can apply the formatting manually again and then use F4.