This is a difficult one, Cossy. Like you, I believe shouting at a child is as bad as spanking, but speaking to the parents again will not do the slightest bit of good.
They doubtless will feel and probably say that the language they choose to use is none of your business and neither is the way the bring up their child.
On this last point, you, I and many others beg to differ, as we do think it is our business if the child is being harmed, or is at risk of harm.
If you decide to alert social sevices, either do so anonymously or ask them not to disclose your name or the fact that you are a neighbour. But I think you need more to go on than frequent crying, parents¨bad language and the fact that they shout at the child, if there is to be any hope that social services can do anything.
DO NOT approach the child's school - they can hardly report something that is only hearsay to social services. But if the child is behind in his speech, the school is presumably aware that there might be a problem in the home. However, they neither can nor should discuss it with you.
I am not sure that the frequent crying is an indication that something is wrong - it could be, obviously, but children of four and even nine and ten, tend to cry these days. We were told at four or five, that we were too big for such babyish behaviour, but this has changed.