Thanks for reaching out – we don't have a specific date or time frame on when it will happen. For now, you can continue using Google Play Music and access all of your stuff from there. We'll also provide plenty of notice before anything changes to GPM. Hope this helps clarify!
PlayStation 4 currently do not support GPM. But, this will be a good idea for YouTube Music (since it will replace GPM eventually). We suggest sending your ideas/suggestions/feedback here: http://yt.be/help/feedback . This will help us prioritize the requests!