I wet sanded my doors with 400 grit and I can assure you that there are no sanding scratches in my top coat. Primer wet-sanded with 400 will actually have a bit of shine and I could not imagine how that smooth surface could have scratches in it deep enough to cause any problem. I don't know much about the current theories but I do have first-hand experience and an relaying my results.
Back in the day we used to paint show cars with metallics and translucent candy colors and the only time we ever used 600 grit was to color sand before rubbing and polishing. The final sand prior to paint was always 400 wet.
But, I'm not doing any fancy finish, just a single-stage, stock solid color. Perhaps if you are doing some show-quality basecoat/topcoat finish you would feel better using 600 or 800 grit, but for those guys reading this who want a stock color 400 wet is perfectly fine.
IMO, of course.
