Four! How are they supposed to learn to ride a bike? I wouldn't be comfortable letting a child ride on the road outside of quiet residential streets until they're at least 8 to 10 (depending on the child).
I cycle to work in summer and am lucky as there are scenic cycle tracks the entire way (converted old railway line, completely away from the roads, it's a very peaceful journey!) and I suppose local kids could learn there... But not everywhere has cycle routes available. A small child on their little bike is totally different to an adult hurtling along. How fast can a 4 year old even be going? They shouldn't be stopping anyone unless what they're doing is genuinely unsafe.
Don't think there's much to worry about around here to be fair. Group of teenage lads hangs around outside my work on BMX bikes, jumping around and doing tricks on and off the pavement, into the path of cars and buses (it's at a large bus stop), playing "chicken" with pedestrians by deliberately riding at them. Also getting into fights with the local junkies
. The police see them plenty and they NEVER do anything about it. Even when they went through a phase of bumping their bikes into the wall of our building, smashing and breaking off tiles. Sigh.