No official numbers, but eye test is in the 4.6 range.
But RB's don't have to be Blazers to be effective, the kid rushed for over 2200yds vs good Broward comp without having world class track speed.
His footwork, vision & balance is what makes him a great RB, he's a natural athlete that plays Basketball & that's where you see him excel at the most is with his footwork & agility to make cuts & transition with fluidity & quickness. He won't beat too many in a straight line race, but in terms of lateral movement & taking angles while,running with a low,center of gravity he's very good at that. Usually taller backs tend to run straight up & are too high, but he actually runs with good pad level & can lower his shoulder to be a bruiser making tough runs.
My comparing for him in his commit thread was Kareem Hunt, neither one is super fast but both are big sized power backs who are highly productive.