Is it true that if a toddler bumps his head pretty hard and there was some damage or concussion, symptoms would most likey show up within 4 hrs?

Head Injuries: Nope. Children must be watched for 24 hours. Not four hours. We watch for nausea and vomiting; we watch for inability to be aroused if the child falls asleep. And then we watch for intolerable pain.

Watch for 8 hrs: It would be extremely unlikely to have significant bleeding, but watch for unusual sleepiness, difference in pupil size, nausea/vomitting. Concussions have variable presentation and symptoms can show up even after 12 hours. You know your child best, and can tell if there is anything worrisome going on.

Probably: Once the growth plates of the skull are closed there is very little room for swelling or bleeding and symptoms develop quickly. In almost all cases the first sign of trouble in a child will be vomiting. If the growth plates are still open then there is a little more room for swelling to occur and symptoms may be delayed but the fontanels (gaps between skull plates)will bulge and become tense.