Why 4? Well, by law, kids under 4 should transport with safety seat.
It is not about virtues, it is about safety.
Many people believe it is a virtue to yield the seats to kids; as I talked about before, it is not safe the kids and there is no priority seat for kids, only for parents who are holding a kid.
It is not only unsafe to the kid, also the adult is occupying an extra space on the bus or metro in Taiwan.
Ironically, these adults usually have to keep telling the children to sit still; so why do not you just sit with your kids?
further reading, https://www.parenting.com.tw/article/5074662