Hi, from my understand UE4 is free to use (Thanks Epic).
However, it’s free for personal/ educational purposes.
When you release a game, and your intentions are for making money (Mo’ money, more problems, as my friend says - Matthew), then you will need to pay a certain amount of your profits to Epic.
I think the threshold is £5000, so when you make more, you pay a certain fee.
Please correct me if I am wrong.