Exactly. Direct flash as the primary light source is horrid at the best of times, but even more so when the flash throws a side shadow.
The pivot and tilt of the SB600 makes it the minimum Nikon flash to purchase, imho.
Well, direct flash is OK when it's used in already bright sunlight to illuminate your subject (as you would do outside skiing). That is to say it is impossible to use a bounce flash outdoors (nothing to bounce off of). An external flash is good for "fill flash" to properly expose your subject while not overexposing your background (again, this describes skiing perfectly).
So, for the stated purpose, the swivel options of the 400 vs 600 are pretty much moot. The ONLY advantage the 600 has over the 400 for outdoor use is its greater strength which may or may not be useful depending on what you think your shooting distances are going to be. Over maybe 12 or 15 ft. the SB600 is going to be noticeably better, I'd guesstimate. Since the snow is going to provide plenty of ambient light you only have to really worry about lighting your subjects faces, etc.
(Well, and manual controls that I'm guessing MOST D40 users aren't going to care about and that you probably won't care about if you are trying to grab action shots - there's going to be enough to do keeping up with the camera settings and the skiers, I'll bet).
Plus, it's maybe half the size of the 600 which could be a factor for lugging it around a mountain. Anyways, no argument that the 600 is the superior flash, but the 400's main drawbacks are at least partially mitigated by the stated use (outdoor winter photography - no bounce possible and shooting in an already very bright scene so only subject fill is needed). Plus, it's advantaged (size and price) could also be magnified by the state use (lugging around a smaller flash, and if you drop it in the snow and it breaks/gets lost forever you're only out $100 not $200).