If a 3yr old accidentally took an extra dose of singular 4mg will he be okay?

In my opinion, since there's no effect on blood pressure, pulse, brain function he shouldn't have any significant adverse effects. However I would like you to call the poison center to get instructions and advice.

This should not result in any long term problems. Watch for sleep disturbance and behavioral problems. If the underlying condition is asthma, review the treatment plan and avoid allergic triggers such as dust, mold and pet dander. Avoid all smoke exposure. Watch for signs of shortness of breath and wheezing. A Flu vaccination I is advised. If coughing, please follow up. A virtual consult is avai.