If you have no real injuries keeping you from doing all facets of training i wouldn’t put yourself in a 400/800 or 200/400 box.
I think looking at the individual requirements for a sub 50second 400m is the best approach.
I.e: Breaking down the speed and endurance requirements, so that each area is adressed in each block, rather than just going for example long to short and doing no speed work for 6 months.