I agree, probably injector. But BAH is right, could be any sort of damage to #4 cylinder or valves.
As long as this is not a customer's truck, and the cont code is on the driver side, you could always swap injectors from two to four to test before you spend any money. You can literally be at the injectors on that side in less than 30 minutes. You will only need a gallon or so of coolant/water mix and two injector oring kits($10 total).
If it was a passenger side injector code, that is a different story. I would buy a new injector to have on hand. Tear down to the injectors and compression test at the same time. I have nightmares about that side.