I think unforunetly some of this will be tiredness from starting pre-school, it does take it out of them a bit.
I own a just 4 year old DS and if it is any help this is roughly what I do. Firstly, I make sure he gets as many early nights as possible and never gets too hungry or the fall out is horrendous.
I am careful to chose my battles carefully, I will let a few things slip but once I have made a stand on something I always follow through.
Some of the time when he gets himself into a state I don't think he knows how to calm himself down, I find a firm cuddle will help and get him to pretend my fingers are candles he needs to blow out if he's in that sobbing, can't breath properly state.
He does get some treats but they aren't linked to behaviour as I think the danger of star charts and pasta charts is that they behave because of what is in it for them, not because it's something they just have to do.
I have used time out with him about 4 times since he was born, he was threatened with it today but sorted himself out before it got to that stage.
I think also they get a whole range of new feelings about the world and their place in it at this age that they find it hard to deal with sometimes so we have regular chats to see how he is feeling, what he has been doing at nursery and what he would like to do.
He is though a complete doddle compared to his older sister so you have my complete sympathy. My worse moment was when she was about 4.5, I was 8 months pregnant, it was the hottest day on record and I turned into a fishwife in the garden, you know the sort where your throat hurts afterwards. When I had finished I heard a rustling from next door and realised that my neighbour had witnessed the whole ugly scene. She is the school secretary of the school my DD was about to start.