I completely believe this. The EX500 was listed as 59hp, with numbers at the wheel in the low to mid 50s. The 400 feels only a bit slower to me (mostly in having more trouble trying to accelerate while in too high a gear). I'm sure the weight difference helps a bit too.
There are also apparently a number of tunes/setups used on the 400 for different countries' licensing regulations. I've seen something listing it as 48hp even.
Just for comparison, here's the 500. As others have stated, it's not a 100% accurate comparison because it wasn't done on the same dyno at the same time, but it gives a point of reference.
I'm still very happy with my purchase as a sidegrade from my 500. I'll occasionally feel like it doesn't have quite as much oomph (like I said, usually when I'm in the wrong gear - an easily remedied situation), but most of the time I'm thinking about how it's just as good as, or even better than, my modded 500. I can't wait to get some springs to match my weight (though they're much closer to correct than the 500's stock springs were).