The "F-you 4s" are a normal, but often frustrating stage of behavior for 4-year-olds. They are often ego-centric and unable to sympathize with other's perspectives or reason through things logically.
Intense emotions can be caused by the toddler not knowing what they want, and being unable to communicate their preferences.
Most of this behavior is a normal part of childhood development, but if your child is causing serious damage to your home or is seriously violent towards others, it may be time to seek outside help.
Read more here (via Insider)