I don't have a 400 but I've done a fair bit of travelling on a XR600 and much of what G600 said strikes a chord.
First the good points:
It's light (compared with most of the old skool alternatives anyway) so it's easier to pull out of bogs / sand etc and you do get (a bit) more tyre life because of the lack of weight.
It's simple and relatively easy to fix at the side of the road. If you've chosen carefully and don't flog the thing to death you probably won't have to do that much anyway.
It crashes well - no radiators to buckle and the things that do bend can mostly be straightened with a hammer.
By travel bike stds the suspension is really great (imho). It's the one area I've never had any problems with whatsoever.
The (600) engine characteristics are well suited to overland travel.
On the other hand:
The subframe is weak and needs quite a bit of thought about how to reinforce it. You'll need to be creative with luggage fitment.
It's stepladder tall so, unless you're equally tall with long legs, it's a pain to get on (off isn't so much of a problems as it's downhill). That's ok when you're full of enthusiasm and adrenaline at the start of a trip but after a long day when you're hot and tired you can really get to hate the bike.
The seat and tank are not great. The seat because it cuts you in half after about an hour and the tank needs to be at least twice the size.
It's kickstart only - believe me there are times when you'd happily trade a few extra kilos for an electric start. The 400 has to easier than the 600 but it's still going to require more mental than physical strength to use it at times on a long trip.
It's very simple electrically. That's a good point mainly but it does mean you'll have to watch what else you run from it. On the 600 I could run either my electric jacket or the lights but not both at the same time.
The 600 in Western Sahara some years ago