Do you realise that "spending" the coins is a taxable event?
No. If you don't convert, you don't need to pay anything at least not in Europe.The 4% is fixed. Even if your savings account pays 0.1% interest you have to pay for a 4% gain. if bitcoin is going up or down 99%, you still pay for a 4% gain. Value is based _only_ on the value at jan 1st, which kind of sucks if bitcoun peaks around newjear.
Short-term hodling is seen as speculating "work" and full income tax (upto 52% here) is due on the actual gains.
Long-term hodling is seen as investment and not taxed, except for the above 1.2% / year.
We still pay 21% sales tax if we spend any money, but conversions between bitcoin and euro's are not counted as spending.