Final answer:
The primary intervention for a pregnant patient with severe preeclampsia and significant proteinuria and reflexes is the initiation of magnesium sulfate therapy to prevent seizures.
Step-by-step explanation:
The priority intervention for a pregnant patient admitted with severe preeclampsia, who has 4+ proteinuria and 4+ reflexes, is to initiate magnesium sulfate therapy. This intervention targets the prevention of eclamptic seizures, which are a major complication of severe preeclampsia. Magnesium sulfate acts as a central nervous system depressant, thereby reducing the risk of seizures. While options such as inducing labor or preparing for an emergency cesarean section might eventually be necessary, the immediate concern is to stabilize the mother and prevent seizures. Pain relief may also be considered, but it is not the priority in this situation.