Yeah, the 400 is an all out sprint, at least with the 800 you can pace yourself a bit.
Ran it once in HS. Us football guys ran with track team. I sub one time. Had no respect for anything beyond 200m b/c I never had to run it. After that one race, my body shut down completely at finish line. All I remember was turning past 200 meters on that turn and I was right there with pack hauling a$$ then last 200 was body folding like a tent, thighs burning inside, back tightening, no oxygen...just falling apart. LOL
I have mad respect for the 400m to this day. My little brother used to kill it in HS, not sure where he got that stamina. My mother was a sprinter through college and she told me she never ran the 4.