Your back is never getting into extension. Might be the 4-inch belt. Take it off and post a video of 225x5 with your back in extension.
The Belt and the Deadlift | Mark Rippetoe
Lower Back Position Control
You have to Learn to Control Your Back | Mark Rippetoe
Low Back Positions - Flexion, Extension, and Overextension | Mark Rippetoe
A New Cue for Setting the Lower Back | Mark Rippetoe