I need some help to understand, how do people in my Level (4) get players
that are 51 stars?

The highest I can buy from Top Eleven is 45 stars (scout player).
The youngest is 22 years. And I sort by most expensive, so I buy the best training potential.

I bought several of those, and I let them play every day, and I train
them hard, and still the best Ive got to is 47 stars (up from 45).

Am I missing something? How do other teams get players to over 50 stars on lev 4?