A lvl 5 trainer will get you around 2-4 extra pops on a player than a lvl 4 trainer. I think you should spend the money on a new player. 2-4 pops shouldn't make much of a difference unless you want to train him for the NT or something. And the salary for a lvl 5 trainer is significantly higher than the salary of a lvl 4 trainer. That is the main difference to consider.
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