Progressive level up.

[oracolus]

This is a good idea except that high level players will become too overpowered to an extent making the runs too easy. Just my opinion

Edit:
Maybe if the levels leading up to Tier 4 had the progressive levelling applied it would be a bit more fair compared to Hero classes becoming better.

Good Point. Perhaps the proficiency or the rate of it goes down as you level up in the higher tiers.