I just reinstalled, after deleting all of my presets and it opened fine. I closed it, then put all my presets back in the user/Application Support/Compressor folder and it won't open again. It appears as if it doesn't like any presets you made with version 4.0. If that's the case, then this update is ****! I do TV spots, and upload to about 20 different stations. They all have their own formats, which I made presets for. I'm not going through this again, like I had to from version 3 to 4!
If anyone finds a work around, please let me know. I prefer Compressor to Media Encoder, but if I have to switch I will. As well as returing the $8K Mac Pro I just ordered!!!