As long as you have your PSN account linked to an Epic account, you will also be able to play on PC and/or Mac. Your items and progression will be available cross-platform. However, while playing on PS4, you will only be able to play with other PS4 players, and while playing on PC/Mac, you will only be able to play with other PC/Mac players.
Furthermore, playing Fortnite on Playstation 4 will lock your Epic Account to the PS4 and PC platforms. This means that you will not be able to play Fortnite on Xbox One while logged into that same Epic Account.
If you still wish to play Fortnite on Xbox One, the only way to do so is to purchase an Xbox One copy of Fortnite and play without linking your XBL gamertag to your existing Epic Account.