I've not owned a 400 but I've had a 600 for a long time and most of the comments seem very (almost painfully) familiar.

The seat isn't really designed to be sat on. It's something just to park your backside on in between standing sessions. On its own I recon you get about half an hour before it feels uncomfortable and about two hours max before you feel like you're being split in half. A sheepskin isn't going to increase that by much. I once did a 2000 mile winter Eurotrip with the original seat and I can still feel the pain now.

The electrics are really rudimentary and lack of a proper battery with its ability to power tent lights, charge phones etc does become a real niggle after a while. There are ways round this - I diy'd a small 12v battery + regulator into the std generator but it's never going to be as good / reliable as a factory fit. The std generator was good enough to power the lights or my electrically heated jacket but not both at the same time. The lights themselves were close to useless, even when I managed to upgrade them to a 55w halogen bulb. Again, not impossible to fix but another area of expensive aftermarket substitution that only deals with half the problem (no lights with the engine off).

The 400 might cope with it better but the "reliability" of kickstarting isn't all it might be. The kickstart lever itself isn't the best bit of design Honda ever came up with and wears (and then snaps) around the pivot. I've had three of those levers and two have shown significant wear. I got thoroughly tired of routinely kickstarting the bike - especially as it has a tendency to stall at lights, road junctions etc. It would idle for ages and then stall the second you put it into gear.

Touring oil life is much longer than the minuscule mileages quoted. I've always worked on a 3000 mile interval and would be happy to extend that with a decent bike friendly synthetic. My rule of thumb has been to change it sooner if it's been ridden hard enough to use oil (high revs, hot temps etc). At normal speeds on open roads it doesn't use much at all between changes. A filter every second change and, as has been said, careful with the filter cover bolts.

Having used the 600 as a travel bike for many years I wouldn't go down the 400 route for a long trip these days as there are better options around, but equally there are worse ones. If you can get a decent one cheap enough and are happy to put the effort into overcoming its shortcomings (the subframe for example) then it it could do the job (= damning with faint praise)