Reading on the Techpowerup review, the 8400 will only clock to 3.8GHz across all cores, and 4GHz on single thread. They was unable to increase the clock speed to 4GHz across all cores no matter what they did.https://www.techpowerup.com/reviews/Intel/Core_i5_8400/17.html
Seems to be a bios 'issue' with the 3.8Ghz on all cores, wonder if it's going to be 'fixed'.
The i5 8400 looks like ridiculously good value otherwise, they really did hit some good price-points with the 8300 and 8400.