Hi, M. -
At 4 it might be hard for her to understand/express what she's feeling and why. Here are a few suggestions:
*Is she getting enough sleep? Maybe she needs to nap. ParentCenter.com offers guidelines on how much sleep a child should get. My 7 year old still needs almost 12 hours of sleep to "function" properly. If she doesn't for one night, it's not a huge deal all the time, but if it adds up, she's a mess.
*Does she like to color? Maybe have her draw you a picture or color you a picture of something that she loves, or a time when she's happy, then have her draw a picture of a time when she's sad, and see if she can explain to you what's going on. That way you get to see what's going on w/o it seeming like you're forcing her to come up with something - it's a lot less confrontational.
*Does she like for you to read stories to her? There's a book called "Alexander and the No Good, Very Bad Day". I'm reserving it for my daughter at the library. You can read the book to her and ask her what she thinks about what's going on in the story, or if she's ever felt the way Alexander feels. I understand she's 4, so her answers aren't going to be eloquent psychological descriptions of her emotions, but maybe it will help give her a springboard to finding the words to explain how she feels. At the very least it might help you get some insight.
*Have you talked to your pediatrician? Maybe they can give some insight.
Maybe what I'm saying is a little over the top for her, but I think the most important thing she needs to know is that it's okay to feel whatever way she feels. Give her lots of hugs, tell her you love her, tell her positive things about herself or give her lots of praise for things she does.
Hope this helps...
R. :)