I think the idea of a 400K is to finish and sleep before the second sunrise. Anyone who starts at 4am should be in bed sleeping well before a second sunset if they want to comfortably do a 1200K. Ona flat 400K, this could mean finishing a 8 or 10 pm but on a hilly one, it could mean midnight or 2 am. If you get in at 2am after 400k on a 600k or 1200k, you can sleep 3-4 hours.
I believe ACP published statistics on finish times and PBP success rates. I also believe the DC Randonneurs or Potomac Pedalers did similarly publish such information but I do not have that anymore. Of course if someone has the ultimate goal of completing a 400K, what I wrote means nothing but a 400K is sort of the first day or maybe just short of the first day in a 1200k (Loudeac is 447km into PBP)