2000 Ford Ranger

I've recently had the engine replaced in my 2000 Ford Ranger. The plugs and wires were replace when the engine was installed. The replacement engine only had 50,000 miles on it, but I've had nothing but problems since it was installed. It is hard to start, idles rough, hard to start, isnt accelerating like it should, is running rich, and feels like it's going to stall when stopped. The check engine light came on and I took it to my mechanic he said the error code showed that the #4 spark plug needed to be replaced. The plug was burnt. The diaognostics and replacement plug cost $53.00 dollars. The car ran a little better (although not much) for about a week, and then the check engine light came on again. Now I'm having the same problems as before, I am going to change the burnt spark plug again, but I'm looking for a long term solution. Have you got any suggestions?L