If I was diagnoze with preeclampsia and I am pregnant can I deliver until 40 weeks? If not why?

Two patients. Ob providers often need to consider the risk and benefit of two patients--the expectant mom and the baby. If the pregnant woman is getting very sick quickly, then she might need to be delivered before the baby reaches term (hopefully after getting steroids if appropriate). If the condition of both mom and baby is stable and the pregnancy is preterm, the OB may discuss 'expectant management'.
Possible. The risk of doing nothing with preeclampsia is that you may have seizures (eclampsia). If you are far from term and your baby's lungs are not mature, you may be monitored so as to extend the time available for the baby to mature. However, if you are near term and/or having severe symptoms related to preeclampsia, early delivery may be warranted.
Possibly. It is simply a balancing act between giving time for your baby to develop and minimize complications associated with prematurity vs preventing any severe symptoms that may occur secondary to preeclampsia.
Depends. Preeclampsia is high BP developing in the 2nd or 3rd trimester and may involve protein in the urine. It's classified as mild vs. Severe depending on your symptoms, lab work and baby's status. If your: BP is very high; your labs are abnormal and concern your ob; or if your baby is having problems, you may have to deliver early. Otherwise, rest and observation may take you to your due date.