 Complete the following phrase:  Prost won his fourth and final title in a year when he was regularly challenged by teammate Hill and by Ayrton Senna. Shortly before the Portuguese Grand Prix in October 1993, Prost announced he would not defend his world title, because the clause in his contract did not extend to 1994 and Senna would be able to join Williams for the upcoming season, and instead opted to retire. At the seasons end Prost held the record for most Grand Prix victories, a record that stood for almost a decade. On the podium in 