Cylinder 4 low compression paired with misfire
I've got a 2003 touring with 101k miles on it. It's got a few mods such as a plenum spacer and exhaust manifold. A few days ago out of nowhere the car started to misfire pretty badly when I started it up. I had it towed to the dealership because I didn't know what kind of stress it would cause to drive it the whole way and they told me that cylinder 4 had low compression at 140 psi compared to the other two on bank 2 which were at 185ish each. They also told me that the spark plug was completely fouled and that the electrode was completely covered in carbon.
So at that point I decided to take the car myself to save money and replace the spark plug and see what happens from there. Well, after installing it, the cylinder was still missing pretty badly. In the past few months I've replaced all my injectors and coil packs, and I put in NGK iridium plugs at 96k, so I thought it was kind of weird that 5K miles later the spark plug is ruined.
Now I'm stumped. The car is still misfiring but I've replaced all the usual culprits. I haven't performed another compression test yet but I plan to whenever I can. As of right now my symptoms are:
-CEL light with code p0300
-misfire in cylinder 4
-low compression in cylinder 4
-white smoke in exhaust
-running rich and strong gassy smell
I'm thinking it's either my head gasket or my piston rings because those are pretty much the only other seals in the cylinder. I need to know what you guys think it is and what I should do to fix it as soon as possible because I'm moving soon and can't take the car with me.