How much independence should a four year old have?

My son is four and absolutely desperate for independence. The other day his sister fell over and, as I was dealing with her, he crossed a small road. I saw him. He looked both ways and walked across. However, as I started to catch him up, he attempted to cross a fairly main road. Again, he looked both ways and began to cross. I shouted loudly at him and he stopped and came back to the pavement. He was really upset and pointed out that it had been clear, which it had. But he is four. I don't feel comfortable letting him cross roads all by himself. I said to him that we would cross the road together, but that he could tell me when it was safe to cross, so that he had a bit more control. 

He often goes into small charity shops with some money, whilst I wait by the door. He chooses the toy he wants, takes it and pays for it and waits for his change. He also unlocks the front door every day when we get home, walking ahead of us and doing it by himself. I like to give him some independence. Small things which he can do that give him a sense of self and help him to understand how the world works.

He has also started requesting to go a different way home to me. This involves him walking along a road parallel to me. I am on the main road and he walks along the back streets. He is alone for about 20 metres until we meet again. Then we cross the road together and he walks alone along the back street for another 20 metres. I have let him do it three times now, but I don't like it. I accept he wants some independence and this seems to be the best compromise I can think of.

What would you do? How much independence do you give your children? I am pretty sure that if we lived in a different area, our parenting would be completely different.