I Have A 4 Year Old Who Almost Killed Her

I have a 4 year old who almost killed her baby brother by sticking a penny and dime into him mouth and he swallowed them, he is fine thank god and she says she didnt mean to hurt him she doesnt know why she did it. I just want to know can a child this young know what they are doing and also is there something wrong with her more then a jealousy of her sibling?