A 4 inch difference isn't much in football, especially when both of said players are so reliant on their speed to be successful at what they do. Will he ever be at the same caliber as Hill? Only time will tell, but he definitely can. What Tyreek Hill has that Harris doesn't is an arm strong enough to throw it to him.
It's not just the height, it's the 15 lbs of weight as well, and it's not like Tyreek is at an ideal weight either for the physical aspects of being an every-down WR.
Again, Deonte is a great player and I hope he exceeds every expectation. I think he can be a high-impact player even if it's not in the mold of a classic every-down WR...there's nothing wrong with that.