4yo quick to anger, slow to forgive

[SweetPotato]

My dd has always been a very serious child, and as she's gotten older I'm seeing that, while her impulse control has improved (re. hitting, etc.) she still just gets SO worked up, SO quickly with her little friends, and is extremely tenacious about hanging on to that anger. She still talks about how one girl stepped on her sand castle last August! I would really appreciate some ideas on how to help her understand that a difference of opinion isn't a huge personal affront- it just seems like a hard way to live, and I'd love to help her lighten up (though I understand that some of this is her temperament, I'd still to help her cope better). Has anyone else dealt with a child like this? I'm hoping that it gets better as she matures and realizes that she has some control over her emotions (ie. that she can think about how important something really is, and choose not to let it be a big deal), but for now it's pretty tough. We don't spend much time anymore with friends who have kids near her age, because she seems to get into conflicts with them so quickly. With older kids she's much more willing to compromise, or to at least allow that they can hold their on opinions-- so she seems like the happiest little angel child with them. Then we run into other 4yos at the gym and within minutes it's "no you're NOT taller than me, and I hate you, you're stupid!"-- oh- and it's the most severe with kids we know well, and with me around-- there seems to be some comfort threshold. She's often very interested in playing with new kids, and then she shows sadness if she's upset, but not the flashpoint anger that she shows with her friends. Advice, please!