I assume you would include in the DONE4 Africa, Asia, North America (but not US) and Europe. This is the only continent combination that includes Asia (since you need to go to ICN) and starts from Africa. There is no possibility of including Asia AND South America without adding North America OR Australia/Pacific as well, hence needing a DONE5.

Maximising miles under such circumstances is difficult. You may fly JNB-HKG-ICN-NRT-DEL-NRT-MEX-YVR-LHR-MCT-LHR-IST-LHR-JNB. However this is far from an "optimal" routing from a mileage perspective, but your only possibility in North America is the JL service between YVR and MEX. All other services involve AA and the U.S.A. IIRC. You may add another YVR-MEX-YVR rotation, but I don't know whether this is allowed (you'd fly YVR-MEX-YVR twice). The experts here will be able to advise you on this

Another possibility, with a domestic Canadian flight purchased on top of the DONE4, would be JNB-HKG-ICN-HKG-BOM-HKG-YYZ//YVR-MEX-YVR-LHR-MCT-LHR-IST-LHR-JNB.