It is entirely possible that Sony is being a dick about cross-platform play now, given the lead they have on the current gen market, that easily allows for it. Also WRT cross-platform play, Sony was there first this gen (that is, to PC) and did it last gen too (again, to PC), when MS showed little interest for it. Not a huge technical issue, but it is there. Thus mods that contain audio would have to be converted. Couple of points - there's at least one technical issue preventing some mods on PS4, it uses a proprietary audio format.