If you get a lean code specific to #4, try swapping that injector with another cylinder and recheck codes. Intake leak can affect just one cylinder. You can monitor fuel trims while spraying intake areas with carb cleaner.
Completely warmed up engine in park at low idle, MAF sensor should report about 2.4 grams/sec of air flow. It's cheap and easy to clean the MAF sensor.
Those injectors have been a problem. The best were in the old ecotec (Bosch which are bulletproof, a real million mile injector), the worst were the Siemens/Deka and these fall in the middle. Luckily they are dirt cheap, you can get OE ACDELCO 2172907 for under $40ea from Rock.
If it has more than 100K on these I would recommend replacing them anyway, the do not clean well.