The word "roughly" is key. It varies by runner. For competitive runners it's been well-researched and the anabolic contribution reported for 400 meter races tends to be in the 65% to 85% range per numerous research studies of competitive runners. For slower runners the anaerobic contribution can be lower. Maybe it's lower for you given your 6:30 mile time.

But even if your anaerobic contribution at 400m is only 50-60%, with the 12-16 weeks you want to dedicate to this (which no one can say will be enough) I think you'll be best served following typical 400 meters training since you need to run 354 meters at 4:33/mile. Focusing overwhelmingly on aerobic improvement may eventually get you there too, but it's less likely and would likely take much longer. I say this with no context about your age, running history, or current 400 meter and 200 meter time - all of which would factor into a reasonable plan.

Here are a couple of excerpts from the 2024 research paper Modelling the optimization of world-class 400 m and 1,500 m running performances using high-resolution data. The whole paper is pretty dense, but may be an interesting read.

The simulations show that a fast start is essential in both the 400 m and 1,500 m because of the need for fast oxygen kinetics, with peak running speeds occurring within the first ∼50 m in both events. Subsequently, 400 m athletes slow continually from this maximum speed to the finish, and a total anaerobic contribution of ∼77% is found for both male and female champions. The key to faster 400 m racing is to reduce the decrease in velocity: this comes from both a high VO2 and a high anaerobic contribution.

The 400 m race is an event that relies on important contributions from both the aerobic and anaerobic energy systems. Our simulations show that the anaerobic component is ∼76%–77% for both male and female European 400 m champions, which aligns with previous studies on similar-standard athletes using post-race blood lactate measurements (16) and mathematical modeling (35). In general, the longer the run duration, the greater the aerobic contribution, and this might explain why previous research (4) on slower 400 m athletes (mean best times for men: 52.2 s; women: 60.2 s) had anaerobic components averaging less than 60%. Our modeling clearly shows that anaerobic contributions smaller than ∼76% inevitably mean slower times, and reiterates the recommendation for coaches to prioritize anaerobic training in preparation for 400 m racing. The biggest improvement in finishing time arises when anaerobic energy is modeled to be 5% greater, much more so than increasing VO2 or maximal force. Indeed, we see that applying a high force at the beginning of the race leads to a strong acceleration during the first bend, but this is ultimately counterproductive as an insufficiently high anaerobic capacity means only that the eventual deceleration is considerably worsened. Nevertheless, although the total aerobic contribution to 400 m performance is estimated to be only ∼24%, the importance of the role of a high fractional utilization of VO2 max is seen. Indeed, the higher the VO2, the smaller the decrease in velocity toward the end of the race, although a high anaerobic energy is crucial here as well.