People get way too caught up in the 40 time. Sure it's nice to have great speed but there's alot more to playing the FS position. Most importantly is instinct. You know where the ball is going and how to get there quickly to make the play. I don't care if you can run 4.2 40 (note to Randall Williams) if you can't figure out the play and take few wrong step then you aren't in positon to make plays. I like to know how many quality FS in the league have 4.4 times.
There are plenty of 4.4 guys these days but most can't play football worth a lick. Watch film and see who can anticipate play and make them. If they look fast enough on the field, don't get caught up in the 40 time.