date of issue of vias has nothing to do as you want to defer at least one term. what matters is the validity period of the issued visa. you will see the date until when the visa is valid on the (visa) stamped page. If it is April (Spring term starts in the first May), you can enter by that date so don't need to reapply for visa. However, you must make sure that your Offer of Admissoin Letter is also valid up to the Spring. Since it is not as you mentioned, you must ask the university to reissue one as Bhaute suggested. The university and the supervisor-to-be may agree on your plan if you have some valid reason why you want to defer your admission for 4 more months. Good Luck!!