Although the numbers do appear to be growing steadily, it is worth noting that four thousand apps over the first two months really don't seem very impressive when compared to competing platforms.
The Android Market is leading the charge against Apple's App Store, with nearly 200,000 apps on offer and about 900 new apps a day. Apple still reigns supreme with more than 300,000 apps and about 1,000 new apps a day. However, the Android market is seeing faster growth and could outpace Apple in the long run.
This doesn't really bode well for Microsoft's troubled mobile OS, but on the bright side at least they're not Nokia.