Faster, one by +1 higher tier maybe an extra (primary) pop or two per season. Doesn't seem a whole lot, but if you take every season of training into account, they add up to quite a lot.
Just remember this:
1. Will a higher level trainer necessary for the player to reach his potential?
2. Are you really in a rush to train them? (you get an influx of new blood every season)
3. Are you gonna build your team around them, or just have nothing to do (maybe sell them later)
Thinking about those would lead to you making the right choice (for your situation) of having just a level 4 trainer, or higher.