Hi pdrmant - children usually have very relevant and real reasons behind their behavior, so it's important to explore all the possible reasons, as you are doing.
I have a four year-old boy who becomes angry and distraught at the drop of a hat and usually without warning. This commonly occurs when frustration is present. It seems he is embarrassed and enraged at the same time upon being corrected or even when he is offered advice.
You initially state that his anger occurs 'at the drop of a hat' and 'without warning', but you also say that a precursor is if he is frustrated about something or when he is criticized/offered advice - are both true, like sometimes there's no evident reason for his anger whilst at other times there was clearly some issue/event preceding his anger? Has his preschool teachers provided any information about what occurs before he becomes angry?