anotherglass I don't think the SN question was unreasonable or offensive.
My 4 year old (almost 5) has no road sense, would happily dash into the road for a stone, etc. She doesn't go outside at all without either a crelling harness (reins for older children) or Maclaren Major buggy. She has SN.
My 3 year old (3.2) can walk nicely beside the buggy, look for cars, see the danger if a cyclist is approaching, and cross side-roads sensibly with minimal supervision.
My 18 month old can walk nicely holding hands, and understands the threat of sitting in the buggy if she starts to wander off.
I never leave my DD1 in the care of anyone without drumming into them that DD1 has no road sense and must be restrained at all times.
It is your responsibility to make sure your DS is safe. So you either leave appropriate safety aids, reins, etc. or they don't go out.
Truly ridiculous that you say your 4 year old 'won't wear reins'. DD1 is 4, has SN, and will writhe, kick and scream at times to avoid her harness. She doesn't go anywhere until it is on, and she knows it.