Expression Web 4 has been discontinued by our good buddies at Microsoft but is still available to download unsupported for free.
I have run into issues a number of times where the program just crashes when you run it. It seems that the problem is caused by damaged files in the user folder. You can delete the folder, which is located at:
Delete the whole Web 4 folder and Expression should behave again. You may have to reconfigure your settings but at least it works again.