Personally, I would suggest a lvl 6, since your trainees starting skills ain't that high and the season is coming to an end. But, having said that, since you're new, I don't think investing in a level 6 trainer is a wise move, so I'd say go with 4:)
What did you train exactly? Position and skill? Also, don't play your trainee's so many minutes. They only need 48 mins per week at one position, if you start playing them 100 and it both kills there gameshape and increases the likelihood of injury.