If tylenol (acetaminophen) doesn't help with a fever for a 4 month old can i give him ibuprofen?

Yes, but...: In 2001 Ibuprofen was ok'd for use in babies as young as 2 months of age. The dose is 4.5 milligrams/ pound of body weight ( 10mg/kg). The concern is a fever in a child that young may be a sign of something more serious. Learn how to take rectal temperatures for your baby. If your four month old has a fever over 101 degrees f (before any medicine) call the doctor or get seen. Call when in doubt.