First thing, let's clarify the country count. Yes, we do count Northern Ireland as a separate country even though it is part of the UK. Some people do, some people don't. For our purposes, if we are going to count England, Scotland, and Wales as individual countries, then we have to count Northern Ireland as well.
Besides which, Northern Ireland has its own currency. Yes, it's the pound sterling, but it is different banknotes compared to the English pound. And it's issued by Ulster Bank in Northern Ireland. It is also not legal tender in the rest of UK... however it is legal currency and easily exchanged at any English bank!