From what I understand, the W400 is an older design than the W300. The W400 also has a directional tread, while the W300 has an asymetrical tread. If you do a search on the forum for the W400 (which I did a few days ago), you should find one or two posts about it.
I just bought a set of 185/55-15H W400s for my Corolla since I couldn't get the W300 in the size that I needed. I found that the dry and wet road handling was really good. The tires have quite a stiff sidewall and there is no tread squirm. As a result, the cornering and highway stability are the same as a summer tire. The only difference is that the W400 has a bit less grip. Also, these tires make a noticeable rumble noise at highway speeds, although it is not overpowering.
Unfortunately, I haven't had a chance to try them out on snow or ice yet, so I can't tell you how they perform in that respect.