You might try this workout. Break the 400m into two parts. Run them with a 60 second recovery between reps. Run the first rep at 400m race pace and the final rep all out. Examples:
200m in 26.0
60 second recovery
200m all out
250m in (whatever that your 400m race pace works out to)
60 second recovery
150m all out
Run two sets. Full recovery... 15-25 minutes... between sets.
You can split this how you want. 300/100, 200/200, 240/160, etc. Don't use a standing start for the final rep. Hit the line already jogging.
One of my friends, an age group national champion in the 400m, runs this workout often. I run it in the weeks leading up to national championships. He's top 5 in the world and I'm top 20 in the world in our respective age groups in the 400m.