5* young player training gains...

DC
Stretching +34
Cardio +76
Practice +111

ST
Stretching +36
Cardio +88
Practice +90

4* youth player (now 5*)
Stretching +42
Cardio +56
Practice match +121

So pretty much all similar now. My conclusion is, the 4* youth player that you sign with tokens are all fast trainers, even at 5*.