Chris, you can check the coil pack with an ohmmeter. A haynes or chilton manual will outline how to do it or i can dig it up for you.
I went through the same deal about two months ago. I ohm checked the coil pack. I replaced the alternator (because I had a spare) and the code came back, swapped two plugs- came back, Swapped two plug wires- came back. Added fuel injector cleaner- came back. Each time I cleared the code with my scan tool.
What finally made it go away was I unplugged the battery for 30 minutes. Don't ask me why, but thats the last thing I did.
When I did forum searches about it, a bad head gasket was the cause for some guys. It wasn't for me and I hope it's not for you.
2004 Ford Ranger Sonic Blue 4dr 4x4 4.0 Manual