Im no expert here but I think you should be working speed endurance more, from what I understand there is a speed component to the 800, and would help the 400 even more. Maybe include a day of true speedwork once a week in addition to what you are already doing, and fpcusing more on speed endurance.
I don’t know how much help this would be, and you might already be familiar with this but I recommend consulting the training methods of Alberto Juantarena Cuban 4/8 runner, might give you some ideas regarding 4/8 training
Something for reference:
https://athleticscoaching.org/UserFiles/File/Sport Science/Theory & Methodology/Speed/Sprints/Juantorena Specific Training for the 400 – 800m Runner.pdf