Just be professional (Score 1)

Your company is within its rights to disable your access. It owns the underlying resources and everything you've produced so far. From the post, it sounds like your slightly incredulous that your company would do this to you, get over it. You gave 4 weeks so spend it documenting and talking with everyone in the company to make sure they have as smooth a transition as possible. When you move on, show good faith by providing a way for them to contact you. If your new job doesn't work out, what you do over the next 4 weeks may be a deciding factor if you want to try and come back.