The 400m is a sprint; the 800 is the hybrid. And although speed training for the 400m seems counterintuitive, it should be the priority.
Relationship between 400 m season's best (SB) and the maximal
How to run 400m faster - Quora
How should a 400m runner be built? - Quora
11.35 100m, 23.00 200m, 53.78 400m. Anything visibly wrong with
What is most important for a 400 meter sprint? - Quora
Can You Believe THIS Is Considered a Sprint!?
How running 400m regularly can benefit your 100m times - PE Blog
400 Meters
400m sprint hi-res stock photography and images - Alamy
Cherry Finishes Fourth in 400 Meters – LSU
The 400 Is a Sprint
400 Meter Training Workouts for Sprinters
Sprint (Running): History, Objective, Events & Techniques