1 and 3 are some type of anemones. 2 is an asterena starfish.
the pic is not great on 4 but combining it with your description, seems like a black sea slug or nudibranch as they are called. Dendrodoris Nigra is the species name "if" that's what it is. Really seems like it. But of course I'm not going to guarantee it without a better pic.
It's diet is said to be mainly sponges. Other than that, it is supposed to be harmless to most anything else. Although, when they die they can release a toxin capable of poisoning other lifeforms. But at such small sizes really don't pose such a threat. It would take a big one to really pose any such threat. I actually had a few very very small ones in an old reef I had. I was always curious about them. They never seemed to damage anything. Then after a few months they all of a sudden disappeared? Perhaps exhausted their food source or were ate by fish?
The ones I had were very small slug like with No shells at all. All black with a white spot on them. A couple may have had more than one spot. But most only had 1 white spot.