S.H.
Kids.... can also fall onto the front, of their face/head or fall off and break their arm/leg.
Your son is 4. He wants one. Sure, head pads and knee pads and what not, are required.
But they can still get hurt.
I know a 7 year old boy... that fell of his skateboard... (was wearing protective gear), and still, fell on his face, lost his front teeth, etc.
I know another older kid, that fell off and broke his arm.
Pads... do not protect against falls. Just certain body parts. But a person can still get... hurt or broken body parts.
At 4 years old, they are going to most probably, just use it as a push toy or they will sit on it and roll themselves forward. Not full on ride a skateboard like a 10 year old, would.
My son is 5. He is very athletic. We have a skateboard. Have had it since he was 4 years old. We have protective gear. My son can stand on it and push with his leg to propel himself and "surfs" the skateboard, and has very good balance. But sure, he has fallen off before... and even my 9 year old has fallen off.
And yes, they do HATE wearing the protective gear. But, it is needed.