I sometimes find that SF4500 does not have enough pull to clear up the paint after FG400. In most of my correction work I always chase FG400 with SF4000 to guarantee being able to clear up what FG400 may not have. This has always given me the results I need even on both of my Jet Black 3 series.
Now SF4500 will add more to the paint after SF4000 if you are chasing every last ounce of gloss.
I remember mike saying that he's never had SF4000 fail at finishing out perfect on any paint. I think for most paint systems that sounds like the product to use. Maybe 4500 if you're trying to burnish the paint on your garage queen or something.