My 4 year old is 80 pounds and has hypertension. Whats going on?

Not normal. Hypertension in children should be worked up. Furthermore, that height/weight is way, way off the charts. Make an appointment to see your pediatrician immediately. This may result in a referral to an endocrinologist.
Good Question! This illustrates that high blood pressure is not restricted to overweight children. You need a medical evaluation to determine the cause. There are many including kidney problems, heart issues and even some tumors.
See a doctor. It is likely he's way too heavy. Htn in a 4 year old must be thoroughly evaluated as it is very likely to have an underlying cause that needs to be addressed.