The difference between a 4.6 and a 4.4 40 is often simply from the technique you have coming off the blocks --- it is a track technique-oriented difference. So... some with good technique in track get great times but are no faster than guys on the football field who are .15-.2 seconds "slower" in the track 40.
With Brandon, his football speed was good... but not special. his track speed, apparently, is special. In any event, he was not an overly effective WR for us. that is not to say that with his size, strength and speed that he could not improve and be a player in the NFL. he might just do it. best of luck to him.
As for mendoza --- one of many head scratchers where you ask "how could an athlete of this caliber not see the field at NU when inferior athletes do?" Who can say... but there are many like this at NU. here too... I hope he gets a shot in the NFL.