Energetic four year old with sudden fatigue and headache as well as 101.8 fever that comes and goes?

Fever and headache : Control the fever and you should see the headache and fatigue resolve. Encourage good hydration and call your dr in the morning. Fever can cause both headache and fatigue.

Humph: A 4 year old can usually complain of other symptoms . Anything else? Fever can cause fatigue and often a headache . If he complains of sore throat or earache he might need to be seen for antibiotics but the odds are it is a viral illness and if you don't give meds you will watch the temp rise and fall on its own. If it goes up and stays up take him to the dr.