Currently there is the UEFA European Championship in football (soccer) taking place and the UK has 3 teams participating: England, Wales and North Ireland. It could have been 4 if Scotland qualified. It is the same for FIFA World Cups, the UK always sends four teams to the qualifications.
The UK seems to be the only country in the world which has 4 football teams even though it is considered as one country almost everywhere.
Why does the UK / why is the UK allowed to send more than one team to international football tournaments? Is it actually the same for other sports?
I already asked almost the same question on politics.stackexchange, and the answers see the reasons in history. However, there are many countries who have a similar history like the UK of smaller uniting states.