My fuel tank started filling very slowly, and I cannot add more than 4 gallons.

Check for a pinched line coming out of you air filter box. That will cause slow filling of tank.

If you cannot fill your tank quickly, as in, the gas nozzle keeps clicking off, it is because you have trash blocking the screen at the bottom of your fill tube. You will need to disconnect the fill hose going into the tank, at the tank, where you can reach the trash and clear the screen.