Unity 4 was released three days ago, on the 12th of November. I installed it as soon as I saw that it was available. Unfortunately, it overwrites any previous version of Unity you have installed, so I lost my install of Unity 3. To make matters worse, I could not find any download link on the Unity3d website.
Thankfully, the official @unity3d twitter account pointed me toward the installer for Unity3D 3.5. Here it is.