What type of statute of limitations will be enforced if I'm sued for copyright infringement?

I downloaded a few music and video files from limewire a few years ago. If I'm sued for copyright infringement would the discovery rule or the injury rule be applied when it comes to the statute of limitations. I downloaded the files about 4 years ago. Should I be concerned that the statute of limitations won't begin until the discovery of the infringement, or am I in the clear? By the way, I live in Pennsylvania. I'm not sure how much the laws vary depending on which state you live in.