The difference between trainer 4 and 5 is not that big but you will have to decide whether an extra pop or 2 per season is going to be worth the extra salary/sign on bonus of a lvl 5 trainer. From the look of your team, you only have the 1 really good trainee. In my opinion its not worth it but that ultimately is your decision.
I would settle for at least a lvl 3 doctor for a D4 team. This will significantly reduce the probability of injury while remain within affordable limits for a D4 team.