DD turned 4 last month, she's always been quite bossy but in the last few weeks she's begun saying horrible things about children who were previously her friends I.e 'I don't want X to be at pre-school. I don't like them. I won't play with them.' She's become very snatchy and has excluded children from her games to the point where they get upset.
She has also started making comments such as 'I don't like fat people. I only like thin people' 
Today I picked her up from the childminder and found out she had been spitting at her best friend and had been quite nasty to some of the younger children. The CM dealt with it and DD apologised but I'm not sure where to go from here.
She understands right and wrong but not when it involves her wants.
She starts school in September so I really want to stop this now. I don't want her becoming known as the mean girl no one wants to play with.
What can we do?