Its ridiculous to find it "worrisome" that freshman would not see the field for the first four weeks and then begin to get playing time in week 5 or so.
They're freshman. They were in HS a few months ago. For the first game of the season, they'd been coached by Pitt's staff for less than a month. Its rare that a kid is ready to play at all in game 1 as a freshman. By week 5, the time that they've been in the program and been coached by college coaches has literally doubled from week 1. The growth over that time in knowledge of the playbook, assignments, body positioning, and many basic fundamentals can be tremendous for a kid who is physically and mentally mature.
It is not surprising at all to see freshman slowly worked onto the field. My guess is that their gettin on the field in week 5 (assuming they get it) is based on a combination of their increased readiness, and the poor play of the veterans. I'm sure PN would rather not play most of them at all this year (he's said so in the past), but his hand is being forced by the poor results of the guys who are playing now, and he is going to do something he'd rather not do. And I think the time the freshman get will be limited at first; and will increase over time.