The harder workouts I have my 400m athletes do are the race model style workouts. Something like run 100m @ race pace walk back 50m, repeat to the finish. Or run 300m @ race pace, 30 seconds rest, then finish the last 100m. 2-3 sets of those are pretty tough.
In general, I typically build my athletes over the years starting with the goal of improving their times in the shorter distances. A 48 second 400m runner that has an interest in the 800m, will probably reach higher levels of 800m faster than a 52 second 400m runner.