The contributions of years 3 and 4 to your final MMath mark (in the usual 10:20:30:40 ratio over 4 years) are calculated as follows.
At the end of Year 3, we calculate your Year 3 mark as the best weighted average of any combination of modules you registered for that year that satisfies the regulations. (In particular, this will be of at least 120 CATS that include 90 CATS of modules from Lists A and C. This may or may not include any MA4 modules you take, depending on your results in those modules.)
At the end of Year 4, we calculate your Year 4 mark as the best weighted average of any combination of modules you registered for that year that satisfies the regulations. (Again, this will be of at least 120 CATS that include 90 CATS of Core and modules from Lists A, C and D; which modules take part in the average depends on your marks in them, but no individual module other than the Core will necessarily appear in the final average.)
Over the two years, you must have REGISTERED for at least 105 CATS of List C or D modules to qualify for MMath.
You should be aware that for the award of MMath with 1st class honours in borderline cases, in addition to meeting an overall mark threshold, the Finals Exam Board would normally require a number of List C or D modules above 70% -- these could be from Year 3 or Year 4. (The word ‘normally’ is there to allow, for example, for the possibiliity of mitigation evidence to be taken into account fairly.) Ultimately, in all cases, it is up to the academic judgment of the Finals Exam Board which degree classification to award, and when considering MMath with 1st class honours it does look at the marks on all MA4 modules across both years as an important factor.
You should also be aware that if your overall mark was in any borderline the Finals Exam Board would consider your performance in all MA4 modules that you registered for across both Years 3 and 4 in their deliberations.
More detailed guidance can be found from the Examinations page.
Note that the marks of all modules you register for appear on your HEAR document (aka transcript of marks), whether they are used in calculating your average or not, and that this document can be seen by potential employers and by postgraduate admissions. Therefore, you are STRONGLY ADVISED not to take modules that you suspect you may not pass, and not to take more than 30 CATS of MA4 modules in Year 3.