After insisting that cross-platform interaction was a bad idea, Sony had a change of heart late last September. For those who wanted to play Fortnite on every possible platform, though, they may have had made new ones just for the PS4, or had their account locked to the console and have had to make a new one for the Nintendo Switch.
Now, you can combine both so that you don’t have to unlock everything twice. Just designate one as your primary account, and items from the secondary one will be moved to it. The moving process will take about two weeks, after which your second account will be disabled. That said, not everything will be moved. Fortnite cosmetics and Vbucks will be moved for example, but other items like Unreal marketplace items and Save The World progress will not. Another caveat is that both accounts must have been used before 28 September 2018 to be eligible – one used to play on a Switch, and another on a PS4. (Source: Epic Games)
