You’re up against a different group of applicants in each application, and also fighting for a different number of spots. Your answers from the supplementary questionnaire are graded and probably put into a rank (from what I gather), but as there are a different group of applicants for the 4 and 5 year course your rank won’t be the same. And as there is a different number of applicants and also spots for each course (4 year generally having a smaller cohort thus actually more competitive than the 5 year), whether you are below or over the “cut off rank” for an interview can be different, thus it being possible to be rejected for one but not the other. Hope that makes sense, just focus on the positives that you did get an interview 🙂