My 4 year old has fever all day should i give tylenol (acetaminophen) or better to wait it out?

Fever: Yes you should give him tylenol, (acetaminophen) motrin, or alternate the 2 if his fever is high. You should have a thermometer.

If OK, let fever go: If your child has a fever, but is otherwise well, drinking fluids, eating some, and generally comfortable, let the fever do its job in helping the body fight infection. If he/she is miserable, not drinking and/or uncomfortable this is a good time to give a fever reducer for comfort, and not to just reduce a number.