Hold on to your temporal tuning fork as I attempt to explain what’s going on here. Minecraft 1.4, the Pretty Scary Update, was due tomorrow. Its pre-release build has been available for testing since Friday, however - long enough for Mojang to identify a hilarious but major issue, necessitating another update. That update is named 1.4.1, and it now has a pre-release build. Minecraft 1.4 meanwhile, the Pretty Scary Update proper, has been delayed until Thursday to incorporate the new update.
Oh, the hilarious but major issue? Just mobs suffocating and falling through wooden floors. Nothing to concern yourself with.
Other issues being sorted before 1.4 goes to meet its public on Thursday include players glitching through beacons, Redstone Repeaters failing to turn off, bats spawning too quickly and, wonderfully, “wet wolves looking way too scary”.