4 grams of APAP is not something you need to worry about, unless you're doing it on the regular. The FDA limit actually used to be higher, it's really a worst-case-scenario minimum limit.
NAC, anecdotally, tastes like shit and can upset the stomach, so ironically it may not be a good idea to pump your friend full of NAC unless he has acute symptoms that clearly suggest liver failure/APAP overdose (10+ gms ingested, painful stomach cramps, pallor, unremittant nausea, jaundice).
Though for reference: the time frame for NAC for apap poisoning is: the sooner the better. Within hours of ingestion is ideal, after 24 hours you are probably wasting your time.
But again, like I said, 4 gms of APAP is not a toxic dose in about 98% of the population, & there is not really a need to panic unless there is clear evidence that your friend is in trouble.
Just tell your friend to calm down and drink a glass of milk or something. Unless he is a chronic daily alcoholic or takes multigram doses of APAP every day, there will likely be no long term or acute effects.