I think we have to be careful with looking at 40 times too closely. Having 4.3 speed is great but if you suck in anticipation or play out of position constantly, you will still look like you are playing slow. Plus, being able to shift your hips quickly without losing speed IMO is more important than a 40 yard sprint time.
Additionally, he might be a 4.6 now, but what happens if he raelly has never trained for straight line speed?
For comparison sake, take a look at the 2009 combine numbers for Tyrann Mathieu. Those numbers don't jump off the page at you. He ran a 4.6 40, had a vertical of 24 and a shuttle of 4.3.
Scout.com: Tyrann Mathieu Profile