i was having a misfire issue on a 2000 rs. so i changed the spark plugs and the cylinder 4 plug was fouled. it ran great for about 200 miles and then fouled again. Im going to replace the wires today but the coil pack costs 150 dollars. is there a way of checking if the one i have is functional.

Well, I sprinkled water on mine and it replicated the problem. So you can try that if the coil is the problem

 

Otherwise, I assume you have a 2.5 engine. If so, the spark plug tube gaskets are common leakers. The oil drips from the tube gaskets saturate the wires which causes misfires.

For more RS specific questions check out http://www.rs25.com if you don't have much luck here.

You can check the coil pack with a muti meter, and a haynes manual. the instructions in there are real simple to follow.