Both behaviours are ones he needs to learn not to act on. But lets look at the bigger picture, this is not a child who is maliciously attacking every child he meets...
I think this is key. This is not an out of control child who hits everyone all the time...he occasionally bats at his siblings hand when he takes a toy away..mom intervenes and explains what is wrong.
This event with the 18 mos old is a different situation. And it was unsupervised so really we have no idea what happened.
I think it is perfectly normal to react with anger when hurt. It was normal for the 4 year old and it was normal for the mother of the 18 month old. This is not the point really though. The point is this mother took her anger to a level that shouldn't be directed at ones own children(shaming and threatening)on a child in her care. This is inappropriate no matter what the child's infraction. If an adult cannot control her impulse when angry why is such behaviour expected of a 4 year old?
I think OP that the hitting problem is something that will go away with time...he will get over the stress of the new baby, adjust as the rest of your family adjusts and all will be over. Just keep a close eye on the sibs as I'm sure you already do and prevent what you can and continue to teach gentle hands and body control. Model gentleness and he will follow in kind.
Meanwhile, get a new sitter for him. I think for the sake of the friendship you need to not depend on her for childcare.