F4 is a great tool, BUT
it only works to repeat the last command used. If what you want is to format, let's say, paragraph #3 to look like paragraph #7 (and you are already typing paragraph #15), F4 will not work... Format Painter will. If you have JUST reformatted p#7, then F4 would repeat on p#3, but if, and only if, formatting a paragraph was the LAST thing done.