I just have to say, I could have written this post. I've actually posted about it on mamapedia before too (recently, actually) so if you want to look at the answers I received, feel free to hop on my page and take a look.
I think this is pretty normal four-year-old behavior. My daughter can be like a light switch too. Totally mercurial sometimes. We do a lot of time outs just so she has some time to calm down. Her feelings are so big right now that she just has to have some time away to settle and then she gets back to her sweet self. I do notice that if she has too many sweets, it's worse. If she's tired, watches too much TV, or doesn't have enough one on one time with me, it gets worse. So, on the days where she's particularly bad, I manage her with time outs, I make sure to connect with her, be silly, dance, joke, read, whatever it is we're doing that day, and if she gets completely out of hand, I give her plenty of warnings and then I take sweets away from her for a whole day. She has a sweet tooth that could conquer Japan, so this is useful for us.
It sounds like for both of our daughters, the opportunity to calm down is the best medicine. I don't tolerate disrespect well, and when she loses her temper, that's what usually happens. So, I try to get a sense of when her temperature is rising and try to calm her down before that happens.
I hope this helps. If nothing else, know that I'm there with you and I don't think there's anything wrong with our daughters. They don't call this age the "effin fours" for nothing.
;) Hilary