Should I be worried that I have no set dissertation near the start of my 4th year?
There's wide variation in when people settle on a dissertation topic.  I don't think you can specify a particular time to panic or not.

However, it sounds like it's certainly time to sit down with your supervisor and negotiate.  You should agree on answers to the following questions:
What exactly are your supervisor's expectations for what needs to be in your finished thesis?  What are the "must haves" and "nice to haves"?  How long will it take to accomplish those goals?  What difficulties could potentially arise, and do you have plans to overcome them, or contingency plans in case of unexpected failures?  When do you expect to defend? What other publications do you expect to produce along the way? How does this relate to the timing of job applications?

In this process, you may have to be more assertive than you have been in the past.  Don't settle for vague answers.  If possible, put a plan in writing and get your supervisor to agree.