I'll go with 4.65 to 4.70. I just don't think Johnson is very fast. But I do think he is plenty fast to have a solid NFL career.
Back in the '90s, the Vikings drafted a slot WR from Alabama named David Palmer. Bama fans still talk about "The Deuce" and all the amazing punt and kick returns he had. The thing is, Palmer himself admitted in an interview that he had never run faster than 4.6 in his entire life. His ability to retain speed while changing direction that made him "play" faster than his 40 time.
Running in a straight line, in shorts, against a stopwatch, with no defenders around or ball in the air isn't a good measure of game speed IMHO. I actually worry more about TJ's concentration drops than his speed. Do that a few times in the NFL and you could find yourself on waivers.