Doran Grant is faster than a 4.5. His speed isn't sudden and he doesn't appear to have a ton of immediate quickness, but he is more of a smooth strider.
FWIW, from http://www.elevenwarriors.com/2013/05/22418/ohio-state-buckeyes-football-40-times-braxton-miller-bradley-roby-ryan-shazier
All-American candidate and cornerback Bradley Roby leads all Buckeyes on the board with a time of 4.26 in the 40. That's blazing, and again, would have been the top time at the 2013 NFL Combine. Quarterback Braxton Miller, second on the list, checks in with a 4.32, with corner Doran Grant matching him. Rounding out the top five are wide receiver Devin Smith at 4.33 and linebacker Ryan Shazier at 4.35.
While those numbers may be a bit inflated, Doran Grant is likely the fastest player on the defense.