Small Engine Problems? Ask an Engine Mechanic for Answers ASAP
You're most likely looking at a good carburetor rebuilding and cleaning. After four years, even without the added gums and varnish from the stale fuel, your diaphragms are probably getting a bit tired.
I would clean and rebuild the carburetor with a full carb kit, replace the fuel filter in the tank, check the fuel lines (replace if damaged or plugged), and put in a new spark plug and air filter while you're at it. I bet it'll perform almost like new.
Here's some more info:
I recommend using fuel no older than three weeks unless you are mixing in a good quality fuel stabilizer. Likely 70% of the problems like this we see are fuel or carburetor related. Today's fuels with MTBE and ethanol are nowhere as stable as the fuels we had a few years ago. I recommend against purchasing fuel from some of the smaller stores that sell fuel for less than the well-known stations such as Chevron, Shell, etc. These smaller companies get a price break on fuel from the distributors because they are buying fuel that is near the 30-day mark. Distributors cannot sell the fuel past that age so they offer a discount to these buyers. Result is that you may be getting fuel that's nearly stale right from the pump.
Because of the fuel situation we see a lot of carburetor problems. If you don't feel comfortable with this kind of repair I would suggest sending it to a reputable shop. If you do the work yourself, take pictures or at least make a drawing of where all the linkages, gaskets, and component parts go. Correct reassembly is critical. Clean every part with carburetor cleaner (soak is better than spray) and blow out all the small holes and passageways with compressed air or use soft tag wire to make sure they're not blocked, them blow through them. Do not get carburetor cleaner on the non-metallic parts since it will cause them to decompose and futher plug the carburetor. Remove the carb cleaner from the metal parts by washing them in warm, soapy water then rinsing with clean water. Dry them thoroughly before reassembling using your pictures or drawings as a guide.
Hope this info is helpful!