Should I be worried that I have no set dissertation near the start of my 4th year? I'm entering the 4th year of my STEM field PhD at a prestigious institute in Canada. My supervisor is a bit terrible; never around, and often with fantastical ideas that are much harder to realize than he thinks. I'm starting my 4th year this fall, and my first two projects have failed. Now, my supervisor has provided me a new project of which I am not too stoked on. It relies on a collaboration with another group, and involves a lot of waiting time. I'm worried that the clock is ticking and I have nothing concrete to work on. I have one 4th co-author paper in a reputable journal, but I don't foresee publishing soon due to a lack of success. Perhaps it's bad luck.

Others who are in or have done their PhD - is this normal to experience at this point? Or should I start to worry about my ultimate success?
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.