Without chronic ingestion of medications that compete for glucuronidation or certain CYP450 inducers like antiepileptics, FOUR GRAMS even as a single dose is virtually impossible to cause any harm.

One gram every 2 hours is 12 grams which is on the lower end of toxic doses.

Despite common belief, concurrent alcohol consumption surprisingly does not increase risk, since alcohol competes for CYP2E1 and reduces the rate of production of the toxic metabolite NAPQI. Similarly for chronic liver disease. The use of NSAIDS (ibuprofen, etc) with cirrhosis is absolutely less safe than tylenol at therapeutic doses.