Injectors are problematic on these engines, so much so that GM redesigned the injection system. You can replace it with the updated parts, it's a chunk of money to do so, though.
It's NOT oxygen sensor, MAF sensor, MAP sensor, or anything like that. None of those things can affect just one cylinder.
You can technically test these injectors if you have a high end scan tool and a fuel pressure gauge. The scan tool test allows the technician to measure the precise amount of pressure drop in each injector, and by comparison, you can see if #4 is failing to contribute. Anything short of this diagnostic procedure is simply guess work.
Techron is a fine product, endorsed by GM, for cleaning these injectors. I would put a bottle of that into the tank and just drive it for a few hundred miles to see if it improves. If not, then more diagnosis is needed. But don't overlook conventional causes: vacuum leak, low compression, burned valve, etc.