This may seem like a rather arbitrary percentage - or even a low one - but trust me, it's actually pretty impressive. Windows Vista was released exactly three years ago, and is currently hovering around the 20% mark. Windows 7 reached 4% of the market in just one month; it took Windows Vista seven months to get there.
Of course, much (more than 95%, actually) of this has to do with the fact that people get Windows pre-installed when they buy a computer - and sadly, the (perceived) dependence on Windows is that strong that most shops see no reason to stock up on Linux machines. While I would love to see a more diverse landscape there, I can't really blame shopkeepers for sticking to what sells. At the same time, the difference in growth with Vista shows quite clearly just how much people did not like Vista.
In something that is surely not surprising to anyone, most Windows 7 users are former Windows Vista or XP users; in other words, Windows competes with Windows. Mac OS X nor Linux saw any major shifts since Windows 7's release; no gains, no losses.