Yeah, CJ2K tells TMZ Sports he coulda ran it in UNDER 4 seconds!!!

Chris says he came into the annual pre-draft event weighing around 200 pounds ... but says if he had shed 30 or so lbs for the drills -- he would've gotten into the 3s zone on that historic day!!!

"I think [I could have run it]. If I would have went to the combine at 165 ..."

So, why didn't the former NFL superstar do it!?!

"I probably would have dropped," Chris tells us ... "Ain't nobody wanting to draft a first-round running back that weighs 165!"

Yeah, good call.

Chris ended up having a stellar career after his ridiculous 40-yard dash time regardless ... becoming one of only seven players EVER to rush for more than 2,000 yards in one season.

In fact, Johnson tells us he's still got it ... the 33-year-old says if he really trained for it, he could still run a 4.2!!!