Is it possible to transfer my character from GTA 5 Online, which I played on XBOX One, to my Playstation 4?
You cannot transfer from one current gen console to another only previous versions. To transfer to the PS4 the character would need to come from either a PS3 version or xbox 360 and this can only be done once. The only way you could transfer from the Xbox One to another platform is if you are going to PC.
A user on Reddit had the same issue and confirmed he was told by support it was not possible