I believe it was something (approximately) like:
lvl 1: 50% speed
lvl 2: 75%
lvl 3: 87,5%
lvl 4: 93.75%
lvl 5: 96.875%
lvl 6: 98.4375%
It would be something like each next level is half the difference with the previous level better than the current level. Of course this is not exact, but I heard/believe this model comes close enough. So the difference between lvl 4 and 5 is ~3% in this model. Over 4 seasons, that would mean a difference of 2 weeks. This might be a little pessimistic so therefore I put 3-6 weeks. Still, that is not an awful lot to pay close to a million for don't you say?