There's no single minimum SAT score guaranteeing admission to Boston University with a 4.0 GPA. BU is a highly selective university, and admission decisions are holistic. They consider your GPA, SAT/ACT scores, extracurricular activities, essays, letters of recommendation, and many other factors. While a 4.0 GPA is excellent and strengthens your application significantly, a high SAT score is also generally expected for competitive applicants.
While BU doesn't publish a specific minimum SAT score, successful applicants typically have scores well above the average. To get a sense of the competitiveness, check the BU website's common data set (often found under admissions statistics) which will give you the middle 50% range of SAT scores for admitted students. Scores below the lower end of this range would make admission much more challenging, even with a perfect GPA.