D.M.
Hi M.!
I have a 4 year old too! I know exactly what you are talking about. What works great for us and had made situations a lot easier to deal with is when there is something that she wants and can't have, I tell her no AND WHY, then she usually throws a fit and I WALK AWAY! I have done my part to tell her why the answer is no and that should be good enough. She eventually sees that I am not paying attention to her and she stops! The fits have gotten shorter and shorter and sometimes there isn't a fit at all! She knows I am not going to feed into her tantrums.
Hope that helps you! :.)
D.