Heads-up for Fortnite account going from PS4 to Switch (not sure about Xbox). You can't so you'll be forced to either buy another battle pass or just not play it on switch.
Easy Ways to Add Friends on Fortnite PS4 (with Pictures) - wikiHow Tech
How to Log Out of a Fortnite Account on PS4 | Digital Trends
You Can Finally Merge Your Fortnite Accounts: Here's How | Tom's Guide
How do I install Fortnite on my PS4? - Coolblue - anything for a smile
SOLVED: When I try to link my PSN/XBL account to my Epic account, it says that my PSN/XBL account is already linked to an Epic account. | Dave Voyles | Software Engineer,
How to Link an Epic Games Account to a PS4
How to merge Fortnite accounts on PS4, Xbox One, and Nintendo Switch | TechnoBuffalo