4 Year Old Acting like a Terrible Two - HELP!!!

Hi mamas -
Have any of you dealt with a late blooming terrible two phase? My DD will be 4 tomorrow, has the vocabulary and thinking and reasoning skills of a 5 year old, but has recently (within the last month) started throwing these awful tantrums. She has always been very well behaved, but lately I don't know how to deal with her. I have tried spanking, holding her and helping her calm down, taking her to her room and letting her throw the tantrum by herself, all to no avail. Today was particularly embarrassing. We dropped off her 16 month old brother at gramma's house in order to go to Supertarget for groc and cupcakes for her bday party at MDO tomorrow. We were not there 5 minutes when she starts disobeying and wanting to hold some items I had in the cart. I told her no, they were going to stay in the cart, she didn't need to hold them. Well, she starts getting loud and saying, NO she wants to hold them, etc. I told her very firmly, if you do not stop disobeying and start listening to me, we are leaving the store and not having cupcakes tomorrow, understand?? Yes mommy, I understand. Well, you can all guess how that turned out. 30 seconds later (no exageration, it was that quick), we are headed to the front of the store to leave. I had to drag her out of the cart to my car with her screaming the whole way. In the car, she would not stay strapped in the booster seat despite my swatting her legs and telling her to calm down (somehow I kept my cool - had to be a God thing). So I strapped her in her brother's car seat instead and got halfway thru the lot when I pulled over and talked to her, offering her another chance (I know, I KNOW I should have followed through, she knew the rules, etc.) We went back in without incident and everything was fine ( I used a different entrance though - that's how embarrassed I was). Sorry this is so lengthy but I feel like such a failure. What have I done wrong?? How can I get through this? These are the kind of screaming flailing arms and legs fits that I am afraid she will hurt herself (she also broke her arm 2 weeks ago - different story and not the result of a fit). Right now I cannot just walk away b/c I am afraid she will reinjure her arm. Why would she start acting like this now? There's nothing new in her life that would cause disruption, and even when there has been (new baby, new house, etc.) she has weathered it just fine. Any advice would be appreciated.
Thanks.