I think it's realistic. If he doesn't run sub 4.48, I could see him falling a bit. Obviously, the Bears would have to trade back and get an early 2nd to be in range even then, but there are very loud whispers that this could happen. The Bears have been fortunate that guys like Fields, Jenkins, Jackson, and Gordon (to name a few) have fallen in range for the Bears to either draft or reasonably trade up for.
Personally, I see Johnston going ahead of JSN, along with Addison and probably Flowers. And I wouldn't be surprised to see Jalin Hyatt go ahead of JSN as well when he runs a sub 4.3 40. I think JSN could be the most productive WR from this class, especially early in his career, but there's a real chance he could be WR5 off the board.