This is from my own experience:
19yo HOF, 50tsp
First season trained with lvl 4 on 2 positions.
Next 4 seasons with lvl 4 on 1 position.
Next 5 seasons with lvl 5 on 1 position.
At 29yo he looked like this:
(http://i.imgur.com/JqYJSo8.png)Even though he lost 22 trainings because of injuries & GS training.
If he was 18yo and didn't miss training he could have ended up with 130+ tsp, probably even more if I trained him 1vs1 more, instead of some other skills.
He also played 69 games for Serbian NT.
My current trainee (134tsp):
(26562677) will have between 135-138 tsp (I'll train him next season too) even though he was pretty average at the time I brought him, and I trained him most of the time with lvl 5 trainer. And he also missed 10-12 training due to injuries/GS/ST.
Those are just 2 examples, as a scout for our NT I've witnessed great progress among young players with lvl 4 trainer.
I believe that it's perfectly fine for new managers to start their training with lvl 4 trainer, than when they reach D3 they could replace him with lvl 5. With a good trainee they could build fantastic players that would play 30+ minutes even at the best bbb teams.