Honestly! What would you do?

My daughter is brand new feisty 4 year old. I had to run to the dollar general. I give her the usual pep talk before we go " im only coming for 2 things, we cant get anything else". I give her the option to walk with me or get in a cart. She choses to walk. So i grab my two things and she is screaming for bandaids. Screaming bloody murder for bandaids. I say no and i walk away..as im wlaking away she runs up behind me and slaps me in my lower back IN FRONT OF OTHER CUSTOMERS . So i turn around and go to grab her AND SHES SCREAMING " DONT HIT ME" in this store. We dont use physically discipline. And only about 2 times can i count has she been tapped on the butt and it was for 2 very serious things. I am beyond words. Im so scared someone is going to judge or think i beat my child. What would you do? What do i do? It seems no matter what i do someone on here has something bad to say about.my parentings. What would you do?