Level 4 is a good trainer, but I think most managers think for the money a level 5 trainer is the best. the level 6 and 7 trainers cost a lot up front to buy. You can probably get a decent salary (18k-25k) level 5 trainer for around 200k. If I was going into USA Div III, I would try to get a lvl 5 guy. Hope to see you there next season.
I haven't seen anyone give a good comparison on training between trainer levels, but there was a link in the forums that gave a good breakdown on how long to hold a trainer based on the money paid for him, and the salary he is paid. It was a little while ago.