I wouldn't be too concerned about it, I'm not an engineer though. I've seen people haul motorcycles in the back of Odys, however I don't know how much one of them weighs, but it can't be all that far from 400lbs. Also the Ody is an 8 passenger vehicle. I think its safe to say that Honda designed it to hold passengers who weigh at least 100 lbs, so that would make the back two rows alone capable of holding 600 lbs.
The only thing I might want to be wary of is putting the weight either ontop of or in front of the rear wheels. I would think it would be best to not have the brunt of the load "cantilievered" behind the wheels. And I think it goes without saying that you need to secure it. Otherwise you would have a 400 lb missle in a crash.
You can look at your owners manual for your year and model to get precise cargo capacities, but as a guideline, they assume each belted seating position can hold a "standard" person at 150 lbs. So if you had the 3rd row up, with all three seats filled, it would be rated to carry 450 lbs worth of passengers. Folding the seat down and putting 400 lbs of cargo in there should be easily within the limits. That's especially true if your second row isn't filled to capacity.
The point about securing it is a valid one, of course. I'd be tempted to leave the second row seats in place even if nobody's going to sit there, because they will act as somewhat of a barrier to keep your bulky cargo behind you in case you have to stop suddenly or if you crash.