I have a 2005 Subaru forester that is mifiring so much that if I put it in gear it cuts off within a few seconds. The misfire is on cylinder 4. I have replaced the plug wires in had I had an obvious break in the cylinder 4 wire and this got rid of the misfire originally. It's my 3rd car so it's only driven once or twice a week and it sat for over 3 months due to the brake lines going out. It ran rough after sitting but cleared up and it's been driven with no problem since until now. The wires were NGK 

Edited September 28, 20187 yr by HASx11

is there any oil on the plug wire boots? might have leaky valve cover gaskets.

Lots of choices)   Bad plug / bad coil / bad injector /  Shorting out , (view in the dark )/ valve adjustment .

Checked  compression ?  Looked at the plug in #4 ? 

Is it throwing a #4 misfire code ? 

My guess, as it temp cleared up with new wires , is the coil. 

Run compression test first.