[Too Many Projects]

If it's that close to done and they are dragging their feet, it's time to either strap on a safety harness and start riding their ass to finish it or take it out. 4 years is 1-2 years too long already for a "qualified" shop to have completed the work.
Too many stories the same as yours about this situation. I got caught in one several years ago too, but after 9 months of him screwing up all the metal work I did to get it ready for him, I showed up one Saturday morning with my trailer and loaded it all up and left.
Give them a timeline of when you expect it done and be there ready to take it out in whatever condition it's in.