There is no cap of 4% per year. If you just continue your violations the EU can continue levying fines and go past the 4% until you comply with the lawful order. I am not sure what the cap is in practice, because as far as I know no company has continued to just ignore EU for more than a few months.
I think the only cap is that at some point, a company has no property left in the EU that can be confiscated and all their managers the EU can get hold of are imprisoned for contempt of court. There is no way that the EU just gives up saying "oh well, we tried".
