Like you said Farah has a great aerobic capacity coupled with a good anaerobic capacity, which is why he can run a fast last lap and is fast from the 1500m and longer. But even the 400 is more aerobic than previously thought, according to a study that examined the aerobic/anaerobic energy system utilization of some "elite level" runners at different distances:
200m run: 29% aerobic; 71% anaerobic
400m run: 43% aerobic; 57% anaerobic
800m run: 66% aerobic: 34% anaerobic
1500m run: 84% aerobic; 16% anaerobic
Note that the ratios are a bit different when comparing a 43sec/50+ sec 400m runners, same goes with the 800m etc. remembering also individual differences.
I´ve red that the 400m WR holder Van Niekerk´s base training doesn´t consist highly lactic workouts, and consists also more of aerobic conditioning (capacity in my terminology). But at some point of the preparation his training consists or some hard long repetitions which can be classified as an aerobic power work. But it´s not as beneficial if you neglect the easy aerobic conditioning in the base phase. It hasn´t ruined his top speed (9.98 100m). To utilize the anaerobic power maximally 400m and up, it demands the AeC to be in balance with the AnC. Otherwise the lactate accumulates quickly forcing you to slow down more rapidly. Not to forget that better AeC allows you recover faster and also do more of the AnP work, which is highly spesific for the 400-800meters.
"Coming from a 200m background, van Niekerk admits he dislikes endurance work. Yet killer sessions and reps of 800m and 1000m have proved vital for his improvement in 2015, though he describes them as “a drag and tortureâ€.
"Running an 800m might take me two minutes, but it is then reassuring to know I’ll only need to run for 43 or 44 seconds for a 400m,†he explains. “I know the training is going to help me for my 400m. As well as the physical benefits of this type of training, it has helped my mindset too.â€
It´s off topic, but isn´t it quite common thinking, when for an example a 400-800m runner is slowing down sooner and or more dramatically race after race, the coach might put in more of traditional, lactic speed endurance workouts and or rest days (might think that the athlete is just tired, could be true sometimes), instead of increasing the aerobic capacity/conditioning work to get the aerobic/anaerobic systems in balance. Excessive AnP work and or racing will bring down both the anaerobic and aerobic energy systems. And you´re more exhausted and slower even after a 100-200m sprint. A recharge period might be needed, taking at least 10 days without any AnP/AeP workouts, emphasizing aerobic conditioning and some short sprints with long recoveries. This period could refresh both the anaerobic and aerobic energy systems. Also some training changes afterward is needed to not to lose the balance again.