If I park weekly, it will generate 1.037 (3,7%) but a month only generates 1.14 (14%).
So if I have 100 coins I will have after 4 weeks 100*(1,037)⁴=115,6 (+15,6%) if I repark each week.
Am I missing something here?!

yes, you forgot to calculate 2 extra days in month, because 1 month it is 28 days(4*7) plus 2 days. Plus you forgot to calculate percents from parking reward from each week.