It really means you pay 30% over the 4% gains, effectifly 1.2% over your capital. ie 1200 for 100K total capital per year.
Hmm. Well if there are any Dutch people here interested in a gay marriage of convenience hit me up. I'll split the tax savings.
Gaazje is correct.
You are effectively taxed 1,2% over your total holdings above 25k euro (below that is tax free). This includes any money on your checkings or savings account, stock value and other investments such as bitcoin.
If your return is better than this 4% you are lucky, if it is worse or you even lost money, you are out of luck and still pay tax over that 4%.