Johnson is an awful KR/PR. Awful. Don't care what he did at college any more, he's proven more then enough times at the NFL level last year that's worthless.
And I've never said "OMG DRAFT HIM OR ELSE", I said he's the kind of player you "do your due diligence on".
You're not going to fill many holes with the 4th round pick no matter what. Go look at any random year and look at the 4th round. I did 2008, for ***** and giggles. Tyvon Branch is a very good player. The 2nd best? Maybe the same Mike McGlynn we were saddled with one season who's a backup center, or maybe Tashard "I'm barely a 3rd string RB" Choice, or even maybe my boy from Kentucky Jacob Tamme, who's really only claim to fame is that Peyton Manning likes him enough to throw him the ball some times. Outside of that it's scrubs. In 2007 you find two pro-bowl type interior linemen in Free and Bushrod, a backup RB in Bush and....nothing. 2006, one of the better years, you find Owen Daniels, Jahri Evans, Brandon Marshall (and a FB in Michael Robinson, a return specialist in Washington, and Gostowski, the kicker). Then a whole lot of nothing. Antwan Randle-El type of an outcome as a fourth round pick is almost, every year, going to be on the 5-7 best players to come out of the 4th round.
If the staff thinks that the upside of Robinson as a KR/PR or a screen type receiver (Chip Kelly loves him some WR screens off of PA) is the best they're going to get in the fourth round, it's not a bad selection. It doesn't have to be the pick, but neither the crappy, crappy return man that Johnson is, nor some of the past failed conversions should make them shy away from him.