What is cylinder 1 on a 2004 4.2 engine?

On a 2004 4.2L engine (assuming you mean a gasoline engine, as there are diesel 4.2L engines as well, and the cylinder numbering varies by manufacturer), cylinder 1 is typically located at the front of the engine, on the passenger side (right side in the US).

However, this is a general rule and can vary slightly depending on the specific make and model of the vehicle. To be absolutely certain, you should consult your vehicle's repair manual or a reliable online resource with diagrams specific to your exact year, make, and model. The engine's casting may also have markings indicating cylinder number 1.