can I refuse to show up for the 4th time to my dentist who couldn't get a crown fit on my pre-molar tooth?

You are the master of your own destiny.  However, if you want your crown, you must appear.  Also, you may have signed some policy agreement stating you would pay a "no-show" fee or something similar.  Nobody can force you to do anything, but some choices carry consequences.

Of course you can refuse to go back to your dentist.  There is no legal reason to force you to return.  However, it will go on record as you abandoning your treatment.  There are MANY reasons why your crown didn't fit.  Some under the labs control, some under control of the materials used, and some based on your dentist's skill.  However, if the crown didn't fit, it was your dentist's ethical and legal obligation to NOT cement the ill fitting crown and make you a new one.  I'm sure your dentist is as disappointed and frustrated as you are, but he is doing the right thing FOR YOU!  My advice is to return to the dentist, who obviously is trying "to make things right" for you.However, if you choose not to return, then it would be appropriate for you to call your dentist and explain why you are not returning and transferring your treatment to another dentist. If your relationship has gone sour, and you no longer have any trust and confidence in your dentist, then that would probably be the best choice for all concerned - both you and your dentist.