You’re going to have to make a new one.
Fortnite is out now on Switch, and not only will it allow cross platform play with Xbox One, PC, Mac, and iOS, but also cross account linking- which means your Fortnite progress on one of those platforms also carries over on to Switch, and vice versa. However, as expected, your PS4 Fortnite account can’t be used on the Switch.
This actually ends up creating problems. I assume you are wondering why, so let’s get started: if you have ever played Fortnite on the PS4 for even a bit, your account is associated with PS4. You can no longer use that Epic account to play the game on the Switch. You’re going to have to make a brand new account, and start over. Your progress and purchases do not carry over. What’s worse, Epic or Sony aren’t letting you dissociate your account from PS4 either, so there’s no recourse.
This is pretty shitty of Sony, and actively anti consumer. I hope this is something they are called out on.