Finishing step after FG400, SF4000 is my vote.
Perfect Finish, according to product label (the version I have), is intended for rotary and spot repair.
However, as many other products we have in detailing people do a lot of use from it, and keep getting awesome results (including using it on DA like the Rupes). Remember the example of Meguiar's M101 - FOAM Cut Compound, being glorified on fiber pads.
Here's what's written in the label of my Perfect Finish:
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1 - SAND down with P2000 abrasive paper, then wipe clean.
2 - Apply polish onto EACH sanded area. Polish on for 10 SECONDS at LOW rotational speed with a Polishing Sponge Orange, TILTING the Machine Slightly
3 - Exerting NO pressure, finish polish for 20 seconds, holding the sponge LEVEL.
4 - Wipe over with a Microfibre Cloth.
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Ok, I'm not a 'blindly follow the instruction' guy, but to me, PF is much more intended for spot repair than to polish an entire car. However, when using it to polish an entire car, it can work very nice as 1 step (many examples here on forum).
Sure, depending on your use and technique, it can work a lot better for you than it worked for me,
fact is I've found it to fill more than described and expected, and this is one of the main reasons I've stopped grabbing it from the shelf.
The defects that looked like vanished away coming back to life after ERASER / IPA is an issue hard to forget.
My routine when polishing with Menzerna includes Eraser to remove polishing oils, then wash the whole car using a 'very strong' soap (yet safe - Meg's Hyper Wash), and inspections after drying shows the results are always 'for REAL'. It's a pleasure applying LSP to squeaky clean finishes...
Again, I had only few experience with Perfect Finish, I could have tried more pressure, more aggressive pad, but I just decided going back to Menzerna and not looking back.
SF4000 is an awesome finishing polish, and if you're looking for a finishing polish, this is it. M205 is another option, HD polish (I don't know HD line but I believe this is the polish from their line, and I hear good things about it as well).
Let us know,
Kind Regards.