On a 2001 Ford Explorer (assuming the 4.0L V6 engine which was the most common), cylinder #4 is located on the
passenger side, towards the rear of the engine. Think of the engine as having two banks of three cylinders each. #4 is the rear-most cylinder on the passenger side bank.
However, it's always best to consult your owner's manual for a diagram specific to your vehicle. Engine layouts can vary slightly even within the same model year due to manufacturing differences.
